Two experiments examined the effects of gonadectomy, photoperiod, and melatonin treatment on food intake, body weight, and carcass composition in male and female Siberian hamsters (Phodopus sungorus sungorus). Gonadectomy caused small decreases in body weight in both sexes but did not affect food intake. In males the weight reduction was due to small (nonsignificant) decreases in all carcass components. Ovariectomy caused a significant reduction in carcass lipid but did not affect lean body mass. A short photoperiod (8 h light, 16 h dark, LD 8:6) caused significant weight losses in all animals except ovariectomized females. Daily melatonin injections (12.5 micrograms sc, 3 h before lights-out) also reduced body weight in males and females housed in a long photoperiod (LD 16:8). The photoperiod- and melatonin-induced weight losses were almost entirely due to decreases in carcass lipid; lean body mass was unaffected by either treatment. Although short photoperiods and melatonin treatments decreased food intake, these changes were preceded by a significant loss of body weight. Thus photoperiod induced changes in food intake may be a consequence of the changes in body weight, rather than vice versa. These findings indicate that photoperiod and melatonin have important effects on adipose tissue mass in both Siberian and Syrian hamsters, even though one species gains weight (Syrian) and the other loses weight (Siberian) in short photoperiods.