To clarify the mechanism of body temperature (T_b) rise following voluntary wheel-run training in rats, we investigated the behavioral thermoregulation in female rats by means of a thermal gradient system. We performed thermal gradient tests before training, 4 weeks after training started, and 2 weeks after it stopped. We also examined the effects on the selected ambient temperature and T_b with quantitative training activity. Nine female rats ran voluntarily in a wheel in the range of 6,545 from 1,665 revolutions/night. The higher the wheel-running activity, the higher the T_b level rose at rest during the daytime after 4 weeks of training. The higher-activity rats selected a higher ambient temperature after the training than before, and the T_b was maintained at a high level under this environment. Our results suggest that the rise in T_b level during rest following the wheel-run training is a regulated body temperature change.<br>