We have shown that circadian changes in leptin concentrations are inversely linked to circulating melatonin levels in the Syrian hamster. The present study aimed to assess the effects of subcutaneous melatonin implants under different photoperiodic conditions: did the implants affect serum leptin levels, body weight, food consumption, and/or testicular weight? Male hamsters were exposed to long and short photoperiods for 10 weeks and received subcutaneous melatonin implants (1 mg in 24 mg beeswax) every 2 weeks. Blood samples were collected every week at midday (1200 hours) or midnight (0000 hours) to assess leptin and melatonin concentrations. Testes were measured to calculate weight. Body weight and food intake were determined every week. Melatonin implants blocked the testicular regression produced by short photoperiods. No differences in body weight were observed among any of the groups. Food consumption increased only with the melatonin implant in the short photoperiod. Serum leptin levels in both photoperiods remained constant. Differences were apparent between noon and night leptin profiles. Noon leptin levels were high (16-18 ng/mL) compared to night leptin levels (8-11 ng/mL) in untreated controls. On the other hand, serum leptin concentrations declined in melatonin-implanted hamsters in both photoperiods. The effects of melatonin on leptin hormone profiles are very pronounced, and melatonin seems to have both physiologically and pharmacologically suppressive effects on leptin production by direct or indirect mechanisms.