Syrian (Mesocricetus auratus) and Siberian (Phodopus sungorus sungorus) hamsters exhibit seasonal changes in body weight mainly by altering their carcass lipid stores. These seasonal changes are triggered largely by the photoperiod. Although both species exhibit gonadal regression when exposed to short photoperiods ("winterlike") daylength), they show opposite body weight changes. Syrian hamsters gain weight, but Siberian hamsters lose weight following short photoperiod exposure. Syrian hamsters prepare for overwintering by increasing energy stored as carcass lipid. In contrast, Siberian hamsters decrease their metabolic mass and therefore require lower energy intake for energy maintenance. In Syrian, and perhaps Siberian hamsters the short day-induced weight changes are exaggerated by high fat diets. Both species show photoperiod-induced changes in body weight without changing their food intake, suggesting a metabolic basis for these effects. In Syrian hamsters, the obesity is not secondary to gonadal regression, whereas in Siberian hamsters, the decrease in body weight is independent of the gonads for males but may be dependent upon the gonads in females. The pineal gland and its hormone, melatonin, are important transducers of photoperiodic signals in hamsters. This is certainly true for Siberian hamsters, in which pinealectomy blocks the short day-induced body weight loss. In contrast, pinealectomy has little effect on short day-induced weight gain in Syrian hamsters. Nevertheless, in both species, the body weight and gonadal changes induced by short day exposure are mimicked by systemic administration of melatonin in long day-housed animals. Thus, for these two hamster species, the same hormone, melatonin, produces opposite effects on body weight but does so by affecting the same carcass component. The target sites of action for the effects of melatonin on body weight change, energy metabolism, and reproductive status are not known. However, the suprachiasmatic and paraventricular nuclei of the hypothalamus are potentially important sites of action. The target site(s) and mechanism(s) of action for the pineal/melatonin-independent effect of photoperiod on body weight in Syrian hamsters are also unknown. This photoperiodic response is highly unusual among mammals in that it is not pineal-dependent. Studies of the mechanisms underlying these body weight changes in Syrian and Siberian hamsters may provide fundamental knowledge about how environmental influences affect obesity and they may also provide insight into the various strategies for overwintering shaped by natural selection.(ABSTRACT TRUNCATED AT 400 WORDS)