Female Sprague-Dawley rats living in basin cages (sedentary rats) under a 12:12 light-dark cycle normally have body temperatures (Tb; measured via telemetry) that vary from a mean peak of 38.1 +/- 0.1 degrees C in the dark to a mean trough of 36.0 +/- 0.1 degrees C C in the light. We have found that if rats are housed in activity wheels, their mean peak Tb in the dark, when they run in the wheels, rises to about 39.5 degrees C. Mean trough Tb in the light also rises, to about 36.5 degrees C, although they never or very rarely run in the wheels in the light. Other rats were rotated through two cycles of wheel-open (WO) and wheel-locked (WL) conditions. During the first WO cycle their mean Tb in the dark gradually rose over the first 2 wk, and their mean Tb in the light gradually fell. In the first WL, mean Tb in the dark fell immediately to sedentary levels, and mean Tb in the light fell more gradually. In the second WO condition, both dark and light Tb rose almost immediately. Since rats in locked wheels have Tb similar to sedentary controls, these results support the hypothesis that steady exercise at night results in an upward resetting of a thermoregulatory set-point during the day.