The physiological demands of weight-pulling dogs have yet to be investigated. Two groups of competitive weight-pulling dogs both underwent two identical pull series 3 h apart. The control group (n 8) was compared with a group fed a rapidly digestible carbohydrate and protein supplement after the first pull series (n 9). Blood was drawn before and after each pull series as well as at 15 and 30 min after the first pull series finished. Biochemistry values remained unremarkable throughout the study in both groups regardless of supplementation or exercise over time. Lactic acid showed mild significant increases post-exercise (2·1 (sd 1·2) mmol/l) compared with baseline (1·4 (sd 0·3) mmol/l; P = 0·03) after the initial pull series. When examining the effects of time there was a significant increase in insulin from baseline (median of 10·8 (range 6·8–17·4) μIU/ml) compared with 30 min after supplementation (17·0 (range 8·1–33·0) μIU/ml) and at 3 h after supplementation (19·2 (range 9·7–53·4) μIU/ml). In the treatment group there was also a time effect, with glucagon being elevated from baseline (median of 100 (range 79–115) pg/ml) compared with 30 min after supplementation (114 (range 90–183) pg/ml) and after the second pull series (131 (range 107–152) pg/ml). Evaluation of each dog's ability to pull the same or greater amount of weight on the second pull series revealed no significant differences. In conclusion, weight-pulling dogs have mild elevations in lactate reflecting little anaerobic metabolism compared with other canine sprinting athletes; hormonal changes associated with carbohydrate absorption are reflected within the treatment group, and supplementation had no effect on performance.