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Environmental cues, including photoperiod and temperature, play important roles in the adjusting of physiology and behavior in small mammals. In order to determine the contributions of short pho-toperiod and cold temperatures to seasonal changes in body mass and thermogenesis in Eothenomys miletus, body mass as well as several physiological, hormonal, and biochemical markers (indicative of thermogenic capacity) were examined in seasonally-acclimatized and lab-acclimated animals. Hereby we test our hypothesis that E. miletus can adjust body mass and thermogenesis capacity to survival in winter or short photoperiod and cold temperatures. The results showed that E. miletus adapted to winter by decreasing body mass, and this change was mimicked by exposing animals to cold tem-peratures and short photoperiod in the lab. E. miletus increased energy intake and thermogenesis and decreased body fat mass and serum leptin levels in winter or under cold temperatures, but not un-der short photoperiod. Protein contents and uncoupling protein 1 contents of brown adipose tissue increased significantly in winter or at cold temperatures, but not under short photoperiod. Together, these data suggest that the observed physiological regulations from the organismal, hormonal levels to the cellular level of E. miletus are critically important and allow E. miletus to successfully overcome the physiological challenges of a cold environment in winter by increasing thermogenic capacity, en-ergy intake and decreasing body mass and body fat mass. It seems that E. miletus is more sensitive to cold temperatures and leptin may play a potential role in seasonal regulation of body mass and thermogenesis. [ABSTRACT FROM AUTHOR]