Rats were subjected to an ambient temperature (Ta) of 33 degrees C for ca. 5 h during the last half of the dark phase for 5, 14 or 28 consecutive days (heat-exposed rats, HE), while control rats were kept at a constant Ta of 24 degrees C. After the heat exposure schedule, the levels of hypothalamic temperature (T(hy)) as an index of body core temperature in the HE were significantly lower than those of the controls for 2-4 h in the last half of the dark phase. The low levels of T(hy) persisted during the specific period for 1, 3 and 6 days after the end of the 5-, 14- and 28-day heat exposure schedules, respectively. These results confirm that, in rats subjected to daily heat exposure for ca. 5 h at a fixed time per day, their T(hy) falls during the period when the rats were previously exposed to heat, and suggest that the duration of the specific T(hy) change observed after completing the heat exposure schedule depends on the length of the heat exposure schedule.