This paper describes differences in reproductive and nutritional conditions that develop within 2 weeks of the transfer of wild-caught female Glossina pallidipes Austen to apparently ideal laboratory conditions. In order to investigate the relative sampling efficiency of baited NG2B traps for female G. pallidipes and the flies' changing feeding behaviour on different days of the pregnancy cycle, samples of female G. pallidipes taken directly from the field and those held in the laboratory for a known number of days after larviposition were subjected to both ovarian dissection (including the measurement of the uterine content and the two largest ovarioles) and fat and haematin analysis. Under the laboratory conditions of slightly lower mean temperature (buffered against diurnal fluctuations), lower activity and higher feeding frequency than in the field, the interlarval period was prolonged, the third larval instar was disproportionately prolonged, the ovarioles and larvae in utero showed differential growth rates, pupal size was increased and the fat reserves of post-partum females was depleted. It is concluded that tsetse are very responsive to the precise environmental conditions in which they find themselves and that absolute quantitative measurements made in the laboratory cannot be applied directly to the field situation.