Body core temperature of rats acclimated to heat given daily at a fixed time falls during the previous heat exposure time. In the present study, thermoregulatory responses of heat-acclimated rats were examined during the specific period. Heat-acclimated rats were subjected to an ambient temperature of 32 degrees C for approximately 5 h in the first half or last half of the dark phase for 14 days while control rats were kept at 24 degrees C. Then the rats were placed in a direct calorimeter and were warmed for 30 min with an intraperitoneal electric heater. Measurements were made twice in the first and last halves of the dark phase. Body warming significantly increased body core temperature in all rats. In the heat-acclimated rats, heat production (M) was significantly depressed during the previous heat exposure time but not during the other period. Body warming had little effect on M in the control rats during either period. The results suggest that rats acclimated to heat given at a fixed time daily respond to an acute heat load with a pronounced reduction of M. However, such a response was observed only during the period when the rats had been previously exposed to heat.