We investigated the effects of environmental light and darkness on thermoregulation during both daytime and nighttime by monitoring body temperature (T(b)) and physical activity of rats using a telemetry system. Experiments were performed in both resting and exercising rats. In resting rats, lights-off during the daytime resulted in an increase in both T(b) and activity. Conversely, during the nighttime, T(b) decreased with the lights-on stimulus despite the fact that the activity was left unchanged. Treadmill exercise (10 m/min) always increased T(b) from the basal resting level. In both daytime and nighttime, exercising rats exhibited a persistent T(b)-rise when lights were on. However, in the lights-off condition at nighttime, the T(b) of exercising rats increased to a level significantly higher than that of exercising rats with the lights-on. Our results suggest that light at nighttime causes the suppression of T(b) in both resting and exercising rats.