When rats are given daily sessions of access to activity wheels, the amount that they run can increase over sessions. In addition to the suggestion that this could indicate the development of an addiction, there are alternative explanations. The present study tested whether running at a fixed time of day (ToD) might allow stronger entrainment of circadian rhythms than running at varied ToDs. In two experiments, Fixed groups of male rats were given 90-min wheel sessions at a fixed ToD during their dark period, while such sessions for Varied groups were given at times that varied over days between early in or in the middle of the dark period. In addition, each experiment also contained inactive controls in order to assess running-induced changes in body weight. Daily wheel sessions were given for 12 days in Experiment 1 and for 18 days in Experiment 2 to younger rats. In Experiment 1 the Fixed groups ran more each day, whereas little increase was found in the Varied groups. In Experiment 2 this difference was found only for rats in the Fixed condition whose sessions were held in the middle of the dark period. Independent of condition, rats ran more in the middle than early in the dark period. As previously reported, the rats given wheel access consumed less food and gained less weight than inactive rats. In conclusion, the results provided support for the proposal that wheel sessions at a fixed ToD entrain circadian rhythms and thus facilitate increases in running.