Following preliminary work conducted in Part II of this series, an attempt was made in the anoestrus of 1958 to provide further information on the conception percentage which follows hormonal induction of a ‘repeat’ oestrus. Two hundred and twenty-one ewes, seventy-six of which were lactating (lambs 8 weeks or older), were available in seven farm flocks. Ewes were of several different breeds and ages but excluded yearling or maiden animals. The trial was conducted during anoestrus over the period starting 26 April and ending 2 August. Treatment consisted of the repeated application of the following injection sequence: three doses of 50 nig. progesterone in oil solution, given at intervals of 3 days, with a dose of 750 i.u. p.m.s. administered 2 days after the final progesterone dose. This therapy was repeated after an interval of 7 days; a 15-day-period therefore separated the two administrations of p.m.s., i.e. approximating to one ovarian cycle.

Rams were joined at the cessation of therapy; in most flocks, one ram was allowed for every ten ewes injected. Two hundred and six of the ewes came in oestrus (93·2%) over the period 2–8 days after the final p.m.s. dose. The oestrous response varied little among flocks, the percentage in oestrus rangingfrom 85 to 100%. One hundred and forty-one conceived at the induced ‘repeat’ oestrus (63·8%) and produced a total of 207 lambs (1·47 per ewe). As a percentage of ewes mating, 68·4 conceived. In none of the flocks did the conception percentage fall below 50; lactating ewes conceived as readily as ‘dry’ sheep. The period during which autumn lambings occurred varied from 4 to 8 days among the flocks.

The results strongly suggest that a much higher conception percentage is likely to follow this extended treatment than follows normal progesterone-p.m.s. treatment. From the practical viewpoint, the technique has the disadvantage that eight injections are necessary and that treatment is prolonged to a 24-day period.

Twenty-nine digestibility trials were carried out on various tropical forages and the results obtained were used to investigate possible prediction of nutritive value from chemical composition.

1. The digestibility coefficient of crude protein could be predicted from the level of dietary crude protein. This prediction could be slightly improved by the incorporation of crude fibre in the expression. Results were somewhat higher than those reported from temperate areas.

2. In order to investigate the above phenomena, a value for metabolic faecal protein was calculated. This was found to be similar to values reported from temperate areas. There was no correlation between faecal crude protein and either level of dietary crude fibre or undigested crude fibre.

3. No significant relationships could be obtained to predict total digestible nutrients from either crude fibre or crude protein.

4. There was a low (non-significant) negative correlation between the content of dietary crude fibre and the digestibility coefficient of organic matter; comparison of this value with those in the literature suggested that the effect of crude fibre in tropical areas was different to that reported from temperate areas.

5. Results obtained were compared with standards of intake and nutrient requirements as recommended by the N.R.C. It was concluded that the factor most frequently limiting was digestible crude protein whilst total digestible nutrients were adequate in the greater part of the samples considered.

Four trials, involving 245 ewes, were undertaken in attempts to reduce the prolonged hormone injection procedure necessary in the application of repeated progesterone-p.m.s. therapy. A continuous-injection apparatus was used as a possible means of rendering progesterone administration simpler. Information was sought on the oestrous and conception response where the first injection of p.m.s. was not preceded by progesterone therapy. Previous studies having indicated that oestrogen-progesterone balance may be important in the development of a suitably receptive uterine environment, two trials were conducted in which oestrogen therapy was combined with progesterone-p.m.s. treatment.

Trial 7. The use of a continuous-injection apparatus was studied in sixty-nine ewes, of several breeds and ages, in four farm flocks. Apparatus were used to administer progesterone in propylene glycol solution over periods of 7–12 days. They were adjusted to give an average daily output of progesterone ranging from 5 to 15 mg., administration being made subeutaneously. In twenty-five the function of these over the planned period of administration was considered satisfactory. A subcutaneous injection of 750 i.u. of p.m.s. was given when the apparatus was removed. Twenty-five per cent of the ewes mated after the treatment; of these, only 25% conceived and produced lambs. The poor response would appear to indicate that low dosage level parenteral progesterone therapy may be much less effective in facilitating oestrus and the conditions necessary for conception than the administration of single doses of greater magnitude at daily or longer intervals.

Trial 8. Fifty-one Clun ewes, of mixed ages and non-lactating, were arranged in two comparable groups. One group (twenty-five ewes) received an injection of 750 i.u. p.m.s. prior to the commencement of progesterone-p.m.s. therapy; the remaining twenty-six animals received progesterone-p.m.s. only. Combined treatment consisted of three doses of 50 mg. progesterone at 3-day intervals, with 750 i.u. p.m.s. given 2 days after the final progesterone injection. In animals receiving two injections of p.m.s., the interval separating such treatments was 15 days. Of the twenty-five ewes receiving prior p.m.s. treatment, fifteen (60%) mated, as compared with 54% of those which did not receive prior treatment. For ewes receiving two doses of p.m.s., the conception percentage was thirty-three, as compared with forty-one in the controls. The results indicate that repeated progesterone-p.m.s. therapy may not be conveniently simplified by a treatment in which the first p.m.s. injection is not preceded, by progesterone.

Trial 9. Eighty-five Half-Bred maiden yearling ewes were arranged in two groups. In one, a single dose of 50 μg. oestradiol was administered 24 hr. prior to commencement of progesterone-p.m.s. therapy; the second group (control) received progesterone and p.m.s. only. In each group, 10 mg progesterone was injected daily for 12 days, and 750 i.u. of p.m.s. administered 24 hr. after cessation of such treatment. Sixty-one per cent of oestrogentreated ewes and 78% of controls mated. There was no significant effect on conception; 33% of those which mated after receiving oestrogen conceived as compared with 41% in controls.

Trial 10. Forty mature, lactating Clun ewes were observed after a treatment consisting of seven daily injections of 25 mg. progesterone, with a single dose of 750 i.u. p.m.s. given 2 days later. Thirty-six ewes mated, and eighteen were injected with a single dose of 50 μmg. oestradiol 3 days after service. Fifty per cent of the oestrogen-treated ewes and 11·1% of the controls conceived. Results suggest that oestrogen treatment following mating may facilitate conception by its action on the uterus.

An investigation was carried out into the fertility, hatchability and chick mortality levels of the progeny of indigenous poultry of Uganda crossed with cockerels of four imported breeds, these being Light Sussex, Rhode Island Red, White Leghorn and Black Australorp.

Altogether 11,888 eggs were incubated and 1584 day-old chicks reared in the production of these data.

The percentage fertility of eggs from females of all four cross-breeds both when mated to cross-bred and pure-bred imported cockerels was significantly higher than that of eggs from the pure Rhode Island Red breed. Among the four cross-breeds, the Light Sussex × indigenous birds gave the highest fertility figures.

The percentage hatchability of eggs from females of all four cross-breeds when mated with cross-bred males was significantly higher than that of eggs from the Rhode Island Red breed and when mated with cockerels of imported breeds was higher, but not significantly so, than that of the Rhode Island Red. Among the four cross-breeds, the Black Australorp × indigenous birds produced eggs with the highest hatchability figures.

Chick mortality rates of the four cross-breeds from 0 to 20 weeks ranged from 15·7 to 20·0%, the causes of mortality being listed.

1. A ‘reference plot’ rotation experiment was begun at Rothamsted in 1956 on a field where grass had grown for many years. The soil contained very little HCl-soluble P and K but much N. The soil had been acid, but was limed to pH 6·5 immediately before the experiment started; this pH has been maintained. Very small individual plots (0·00128 acre) were used and nearly all work was done by hand. The results showed that such small plots can be used to do complicated rotation experiments satisfactorily provided that all work is done carefully and movement of soil over plot boundaries is prevented.

2. Responses to combinations of N, P and K fertilizers and farmyard manure (FYM) were measured on wheat, kale, barley, grass-clover ley, and potatoes grown in rotation. Two rates of N were tested. A strip of permanent grass had the same treatments as the arable crops.

3. Winter wheat yielded most dry matter, but wheat yields were increased least, proportionally, by added nutrients. Potatoes gave the smallest yield without manure and responded most to nutrients, yields being increased almost sixfold by NPK fertilizer plus FYM. Kale, barley and grass-clover ley were intermediate in their unmanured yields and in their responses.

4. All crops except the grass-clover ley responded to N; kale responded most and wheat least. Potatoes kale and barley responded roughly equally to P and needed this nutrient more than did wheat and the ley. Potatoes responded most to K, yields of wheat and clover ley were also small without K, but barley and kale needed K less. Interactions between nutrients were large with most crops, particularly with N on kale and K on potatoes, wheat and clover.

5. FYM greatly increased yields of all crops but responses were less when fertilizers were also given. Potatoes responded most to FYM whether or not fertilizers were also used. FYM also caused large increases in yields of grass-clover ley, kale, and permanent grass.

Albumen quality, shell thickness and incidence of blood and meat spots in eggs from the progeny of indigenous poultry of Uganda crossed with four imported breeds were compared with results from the pure imported Rhode Island Red breed and discussed relative to figures previously obtained from indigenous poultry. The measurements were taken for a 5-day period during the 12th month of age of the birds when maintained under a high environmental temperature in Uganda.

The albumen quality of all four cross-breeds, as measured by Haugh units, was significantly higher than that of the Rhode Island Red breed.

All four cross-breeds produced eggs with shells significantly thicker than the Rhode Island Red, only slightly below the generally accepted figure of 0·013 in.

The incidence of both blood and meat spots was lower in all four cross-breeds than in the Rhode Island Red, in the case of blood spots significantly lower in one cross-breed and in the case of meat spots significantly lower in three cross-breeds.

This trial concerns the induction of pregnancy in two flocks of Clun maiden yearling ewes and involves seventy-six animals. Previous observations suggested that treatment of this particular category usually gives rise to a higher conception percentage than is found with dry post-partum or lactating sheep. The trial sought to elucidate some factors of animal and environment which may be associated with this response.

In flock 45, during the period 10 February to 29 March, observations were made on the incidence of spontaneous breeding activity in forty-seven ewes; hormonal therapy was first applied at the latter date. Treatment for all ewes consisted of three doses of 50 mg. progesterone (in oil) given at 3-day intervals; a single dose of 750 i.u. p.m.s. was administered 2 days after final progesterone.

Nineteen of the forty-seven ewes (40%) in flock 45 showed spontaneous oestrus in February or March. Three (16%) of those ewes previously mated came in oestrus after treatment; forty-nine of the fiftyseven unmated ewes (86%) showed oestrus 3–5 days after the end of injection. Of the nineteen sheep that mated spontaneously, ten (53%) produced autumn lambs. Twenty-six of the fifty-two sheep (50%) served in response to the progesterone-p.m.s. therapy conceived. 1·25 lambs were born per ewe that lambed. Some evidence for total embryonic or foetal loss in the ewes during pregnancy was obtained. Results indicate that conception in these maiden ewes may have been facilitated by the presence of corpora lutea or recent luteal activity, which maintained the oestrogen-progesterone balance necessary for providing an adequate uterine environment. For this reason, maiden ewes of breeds with short breeding seasons (e.g. Scotch Half-Breds) may not give such satisfactory response when injected at times comparable to long season breeds, such as the Clun.

1. The amounts of N, P and K recovered by five arable crops and by permanent grass from soil alone, and from fertilizer and farmyard manure (FYM) dressings were measured. All the crops responded well to P and K fertilizers and all except clover responded to N. Uptakes from soil alone are therefore the maximum amounts of each nutrient that each crop could remove when supplied with other fertilizer nutrients (the exchangeable Ca and Mg in the soil were adequate).

2. Permanent grass (free from legumes) obtained about 114 lb. N/acre each year from soil and other natural sources; winter wheat obtained 1001b. N, kale and potatoes about 80 lb. N and spring barley only 57 lb. A 1 year ley of clover and grass fixed at least 1 cwt. N/aere/year. Permanent grass removed most P from soil (17 lb./acre a year), potatoes removed least (6 lb. of P) and other crops intermediate amounts. Most K was taken from soil by kale (70 lb. K/acre/year) and least (20 lb.) by potatoes. Annual variations in the amounts of nutrients recovered from soil by any one crop were much greater with K than with N or P.

3. Most fertilizer N was recovered by kale and least by potatoes; with these crops two-thirds and one-third respectively of the light dressing was recovered, percentage recovery from the higher rate of N tested was less. Kale and the 1 year ley recovered nearly one-quarter of the P applied, permanent grass recovered little more than one-tenth. Clover-grass ley recovered most fertilizer K, apparently taking up four-fifths of that applied. Potatoes, kale and permanent grass all recovered more than half of the fertilizer K given, cereals were least efficient although both responded well to K dressings.

4. Farmyard manure supplied large amounts of nutrients to all crops. Similar amounts of N, P and K appeared to be recovered from FYM whether or not NPK fertilizer was also used. A rough estimate was that crops like kale, potatoes and permanent grass, which received FYM each year, recovered about 30 lb. of N, 4 lb. of P and 75 lb. of K from a 10 tons/acre dressing.

Data on egg production, egg weight, food consumption, broodiness, mortality and body weights of the progeny of indigenous poultry of Uganda crossed with Light Sussex, White Leghorn, Rhode Island Red and Black Australorp breeds were collected collected over the 25-week period from 161 to 336 days of age and compared with the performances of indigenous poultry.

In comparison with the indigenous birds, the cross-breeds had a 61% higher mean egg production rate, a 34% lower food consumption per pound of eggs produced, a 15% higher egg weight, a 30% reduction in mortality, a 23% increase in body weight and a decrease in broodiness.

All the cross-breeds were superior in productive respects to the indigenous birds and the White Leghorn and Light Sussex breeds appeared to be the best for use in the upgrading of indigenous stock for highest egg production results.

Field trials on hexoestrol implantation were conducted in the Waikato area of the North Island of New Zealand during 1958–59, involving 260 two and three year old Aberdeen Angus steers from nine farms. Two levels of hormone implantation were studied, namely, 30 and 45 mg. The trials ranged in duration from three to five months with an average of four months.

Pronounced differences were apparent between farms in the overall growth rates and carcass quality scores of the cattle.

Hexoestrol implantation led to a significant increase in carcass weight, estimated at approximately 22 lb., with little evidence of real differences between farms in the average response to the hormone.

Response to the higher as compared with the lower dose of hexoestrol varied markedly among the separate farms. In general, the lower dose of 30 mg. tended to yield the greater increase in carcass weight.

A small, but consistent, depression of carcass quality scores was manifest under hormone treatment, with no real differences between the two dosage levels.

1. A balanced factorial experiment was carried out with 48 Merino ewes. There were two levels of each of the four factors: environmental temperature, thyroxine status, vitamin A intake and progesterone status. Each ewe was killed at 25 days' pregnancy. The thyroid was removed, weighed and examined histologically.

2. Thyroid weight in the hot-room animals was significantly decreased by large doses of vitamin A. It was not affected by any of the other treatments.

3. With respect to the number of follicles per unit area of cross-section, the following interactions were significant: temperature × vitamin A; temperature × progesterone; temperature × vitamin A × thyroxine. The directions of the effects of vitamin A and of progesterone were reversed by the higher temperature.

4. There were no significant effects on nuclear height in the follicular epithelium.

5. The frequency of marked ‘scalloping’ at the edge of the colloid was significantly higher among those ewes receiving thyroxine injections than in the remainder. This feature was not significantly affected by any other factor.

1. From 1955 to 1957 a cattle grazing experiment was carried out on lucerne and cocksfoot pastures. Hereford-cross steers were used. Results obtained on the assessment of herbage intake either by the use of chromic oxide and faecal index methods or by herbage sampling are presented here.

2. Except in 1955, when techniques were being developed, the digestibility of the herbage on offer to the bullocks was estimated by cold storing herbage, cut with an Allen Autoscythe and later feeding it to sheep in indoor digestion trials. The in vitro digestibility of herbage samples cut in 1955 and 1956 was also determined.

3. In 1956 the mean intake of organic matter (lb.) per 100 lb. live weight by faecal methods was 2·38, 2·07, 2·20 and 2·29 on pastures of lucerne, cocksfoot, lucerne/cocksfoot (broadcast) and lucerne/cocksfoot (alternate 1 ft. drills), respectively, and 1·99, 1·76, 2·02 and 1·97, respectively, in 1957.

4. In 1956 the mean intake of organic matter (lb.) per 100 lb. live weight by herbage sampling methods was 2·14, 1·70, 2·11 and 1·99 on the same pastures of lucerne, cocksfoot, lucerne/cocksfoot (broadcast) and lucerne/cocksfoot (drills), respectively, and 2·49, 1·32, 2·35 and 2·03, respectively, in 1957. The modified Tarpen trimmer, cutting to within 1 in. of the ground was used for sampling in 1956 and the Allen Autoscythe cutting to within 2 or 3 in. of ground level in 1957.

5. Detailed botanical separation of herbage samples showed that the bullocks were selecting the more digestible parts of the plants; the top 4 in. of lucerne and the ends of leaves and stems of cocksfoot. This and other factors which could affect the estimates of digestibility of herbage eaten by the cattle are discussed together with possible sources of error in the herbage sampling methods.

The effects of four five-course and six four-course rotations on some physical and chemical properties of a sandy loam overlying London Clay are reported.

Periods under ley increased the water stability of the air-dry soil aggregates, the resistance of moist soil aggregates to simulated raindrops, the rate of drainage under 60 cm. of water tension as measured in the laboratory, decreased the draw-bar pull during ploughing and the force required to crush air-dry aggregates. These effects increased with the number of years under grass.

After 2 years of arable cultivation the effect of a 3-year ley on water-stable aggregation was still apparent whereas the effect of a 2-year ley had largely disappeared, and those of a 1-year ley had completely disappeared.

A 2-year ley in a five-course ley-arable rotation was sufficient to maintain the total soil nitrogen at about 0·16%. With one year of ley in a four-course rotation a slow decline in nitrogen was found when the initial level was about 0·21%.

The rate of CO2 formation in soils of the arable break was proportional to the number of years they had been under ley; the longer the period the greater the volume of CO2 produced.

Evidence is presented that fertilizer nitrogen is used most efficiently by cereals growing in soil in good physical condition. This is usually best following a period under grass.

Self-diffusion coefficients (D) for Rb+ have been measured in a heteroionic Lower Greensand soil over a wide range of moisture contents at three different compactions. The moisture content is shown to have a considerable effect on the value of the self-diffusion coefficient, a rapid rise in the value of D taking place between 5 and 10% moisture content. It is shown that this effect is not simply due to the increased volume of water-filled pore space at higher moisture contents, and other possible factors which could influence the shape of the diffusion coefficient/moisture content curve are discussed. Diffusion coefficients for Rb+ in eight other soils at pF 2 are also reported, but no simple correlation with any of several soil properties measured could be found. A value of 1·0 × 10−7cm2/sec. would seem to be a reasonable generalization for the self-diffusion coefficient of Rb+ in moist calcium-dominated soils.

1. A balanced factorial experiment was carried out with 48 Merino ewes. There were two levels of each of the four factors: environmental temperature, thyroxine status, vitamin A intake and progesterone status. Each ewe was killed at 25 days' pregnancy. The ovaries were removed, weighed and examined histologically.

2. There were no significant treatment effects on ovarian weight, corpus luteum diameter, lutein cell frequency, lutein cell nuclear diameter, the number of Graafian follicles exceeding 0·9 or 1·9 mm. diameter, or mean follicle diameter. The results suggest that with more animals heat might prove to depress ovarian weight and vitamin A to increase ovarian weight, corpus luteum diameter and the number of large follicles.

3. There were no significant differences between the mean values for those ewes with live embryos and those with dead ones with respect to any of the above indices.

4. The results support the conclusion that variations in embryonic viability due to the experimental treatments were not mediated primarily through variations in ovarian hormone secretion.

1. A series of three experiments were performed to examine grass as a food for dairy heifers.

2. Indoor feeding resulted in lower intakes of herbage and slower growth but gave greater outputs per acre of animal days and live-weight gain than grazing.

3. The maximum consumption and growth rate, obtained on young grass, were 2·84 lb. herbage dry matter per 100 lb. live weight per day and 2·2 lb. live-weight gain per day for grazing stock. The corresponding figures for indoor feeding were 2·49 lb. herbage dry matter per 100 lb. per day and 1·34 lb. live weight gain per day.

4. Live-weight gain per day was linearly related to herbage dry matter intake from 1·48 to 2·48 lb./ 100 lb. live weight/day.

5. Intake of herbage and growth rate only fell after the herbage came into full flower.

6. Increased stocking rate had small effects on growth rate because the animals grazed the paddocks more heavily.

7. 2·6–3·0 lb. grass dry matter offered per 100 lb. live weight per day was judged an adequate ration for yearling heifers. This gave growth rates of 1·60 lb./day on herbage up to the flowering stage and a consumption of 88% of the available herbage.

8. As an experimental technique zero grazing was found inadequate to replace grazing since it fails to reproduce similar intakes and rates of production. It permitted the reduction of intake to sufficiently low levels that live-weight gain per animal and per acre were reduced.

9. The greatest output per acre was found to occur between limits of intake per day of 1·8–2·4 lb. herbage dry matter per 100 lb. live weight. Above the point of maximum output per acre level of feeding had little effect on production per animal and rate of stocking controlled output per acre. Below this point level of feeding controlled production per animal. The case is advanced that these latter levels of feeding are necessary in grassland experiments.

Laboratory indices of P availability were compared for six soils. Phosphorus soluble in sodium bicarbonate, ammonium fluoride and citric acid solutions, and in water was determined. Maximum P adsorption capacity was calculated from data by a batch method obtained from equations using the Langmuir isotherm. Adsorbed P values were derived from the same equations. Measurements of P adsorption obtained by leaching soil samples with a phosphate solution yielded asymptotic curves which also allowed the maximum adsorption capacity to be calculated. Results obtained by the two methods were very similar. Leaching of soil samples by water was the basis for calculating maximum P desorption values; these agreed well with the adsorbed P values obtained from the Langmuir isotherm.

Phosphorus uptake by plants, determined in a greenhouse experiment, was correlated with the various data and parameters obtained in the laboratory; there were no significant correlations with the P values from extraction methods, but there were good correlations with some of the parameters from the equilibration and leaching experiments.

The methods proposed by Pereira for unirrigated coffee have been followed in a study of water use by this crop for the 12 years 1950–62. Soil sampling has shown that the procedure provides a reliable measure of evapotranspiration from unirrigated coffee.

A modification is proposed which has enabled the rate of water use by irrigated coffee to be followed satisfactorily for the 5 years 1957–62. Occasional soil sampling showed that the modified procedure provided an estimate of the soil moisture status sufficiently reliable and accurate for the regulation of irrigation timing and amount.

Irrigation to maintain a favourable soil moisture status has had a marked and beneficial effect on the production of Grade ‘A’ coffee in dry years.

The mean daily rate of evapotranspiration from irrigated coffee is estimated to range from 0·15 in. in hot weather to half this rate during the cool and cloudy season.

The maximum computed annual totals of supplementary irrigation which were required over a 12-year period ranged from 5·5 in. to 31·5 in. with a mean value of 16·0 in. per year.

1. A balanced factorial experiment was carried out with forty-eight Merino ewes. There were two levels of each of the four factors: environmental temperature, thyroxine status, vitamin A intake and progesterone status. Each ewe was killed at 25 days' pregnancy. The embryos and their membranes were weighed and examined histologically.

2. There appeared to be no treatment effects on the embryonic length/weight ratio. The ratio between embryonic weight and ‘conceptus minus fluid’ weight may have been affected. There was no evidence of treatment effects on the degree of differentiation of either the embryo or the chorioallantois.

3. Surviving embryos tended to be large where thyroxine was supplied, i.e. where viability was good, and vice versa. Dead embryos fell into two groups: those less than 3 mm. long and those 6 mm. or more which had probably died shortly before the ewes were killed.

4. There were no significant correlations between the weights of live embryos and mean rectal temperature, plasma vitamin A concentration or any of the histological indices measured in the endometrium. In the hot-room live embryo weight was correlated with the height of the ewe's bladder epithelium (P < 0·01).

5. It is suggested that in the absence of adequate maternal thyroid hormone (i) the trophoblast may enlarge too slowly to stimulate adequately some response in the endometrium essential for maintenance of the embryo, or (ii) disproportionate growth in different parts of the conceptus may result directly in embryonic failure.

1. Rectal temperatures and respiratory rates have been determined in four calves during 5 hr. exposures to ten different hot environments in a climatic room.

2. Among the various parameters based on rectal temperature response, final rectal temperature proved the best for differentiating between the heat tolerance of individual animals. Under conditions of severe heat, where rectal temperature rose almost linearly with time of exposure, tolerance time proved equally suitable.

3. Since initial rectal temperature tended to parallel final rectal temperature, the increase in rectal temperature during exposure did not vary significantly between the animals. Increase in rectal temperature was therefore considered a less suitable measure of heat tolerance.

4. Both the lowest and the highest panting rate were associated with a low heat tolerance. From this, as well as from other observations and considerations, it was concluded that respiratory rate, either alone or in combination with rectal temperature, was an inadequate measure of heat tolerance.

5. A state of high heat tolerance tended to be associated with only a small loss of body weight during exposure to heat, but showed no relation to body weight itself.

6. The order of heat tolerance of the four animals was essentially the same in each of the ten hot environments investigated. It was inferred that within the range of temperatures employed in this study the degree of severity of the heat stress was not an important factor in the discrimination of heat tolerance.