Wheel running was previously shown to influence body mass and torpor in short day-acclimatized Djungarian hamsters (Phodopus sungorus). To determine whether the exercise-induced effect on body mass depends on the annual phase, hamsters were exposed to the natural change in photoperiod and given access to a running wheel (RW) either before, in the middle of, or at the end of the descending body mass trajectory during seasonal acclimatization. Due to wheel running the seasonal weight cycle was prevented or aborted by abruptly rising body mass, resulting in a weight appropriate for summer despite exposure to short days. Torpor was inhibited and testicular recrudescence was advanced compared to controls. In contrast, the change into winter fur remained unaltered. Analysis of body composition and plasma leptin revealed a low body fat mass in RW hamsters not only in winter but also in summer, suggesting a lack of seasonal adiposity. Chronic leptin infusion in winter only decreased body mass in RW individuals although their relative body fat mass probably was even lower than in sedentary hamsters. A constantly low body fat mass is conceivably reflecting an exercise-dependent change in metabolism, consistent with increased bone mineral content and density in RW hamsters. Additionally, bone area was increased, again supported by elongated vertebral columns. Together, the results show a striking effect of wheel running on body composition and the seasonal pattern of body mass and they suggest that the photoperiodic regulation of body mass is regulated differently than the reproductive and pelage responses.