The thermoregulatory ability of pregnant and lactating female rats subjected to terminal heating was studied. Female Sprague-Dawley- rats were divided into groups and placed in individual glass chambers maintained at 39.5 degrees-C. Colonic temperatures were monitored continuously using a rectal probe. Evaporated water was measured at 15 minute intervals, and survival times were recorded. The groups consisted of nonreproducing rats, pregnant dams at day 11 of gestation, pregnant dams at day 20 of gestation, lactating dams at 21 days postpartum, and heat acclimated lactating dams at 21 days postpartum. Rats were classified into one of three thermoregulatory types based on their colonic heating pattern. Type-I females exhibited a linear increase in colonic temperature. Type-II rats showed an initial rapid increase in core temperature, followed by a convexity in the heating curve which persisted until death. Type- III rats exhibited a three stage heating pattern which was similar to that of Type-II animals except for a dramatic terminal temperature increase. Evaporative water loss was closely related to heating patterns in all groups. In comparison to control rats, the thermoregulatory ability of lactating rats was reduced, and there was some impairment during late gestation. The impairment in lactating dams was related to their inability to sustain an adequate rate of water loss. The authors conclude that there is a marked influence of reproductive status on thermoregulatory capacity in female rats.