Serum leptin concentrations were obtained from male Siberian hamsters (Phodopus sungorus) and golden hamsters (a.k.a. Syrian, Mesocricetus auratus) housed on long [light:dark (LD) 16:8] and short (LD 6:18) photoperiods for 10-11 weeks. Blood samples were collected at 45-min intervals for 24 h from individual animals using an in-dwelling atrial catheter. In Siberian hamsters, exposure to short photoperiods as compared to long photoperiods reduced body weight (32.5 +/- 1.5 vs 47.7 +/- 1.1 g) and leptin (24-h mean: 5.3 +/- 0.4 ng/ml vs 18.6 +/- 2.1 ng/ml). Although photoperiod influenced the temporal distribution of leptin in golden hamsters, the main effect of photoperiod on leptin levels in golden hamsters did not reach significance (24-h mean: 7.1 +/- 1.0 ng/ml vs 5.1 +/- 0.8 ng/ml.). Body weights of golden hamsters did not vary significantly following exposure to short photoperiod for 11 weeks (178.3 +/- 3.6 g in LD 6:18 vs 177.8 +/- 7.3 g in LD 16:8). There was no nocturnal increase in serum leptin in either species. Marked interindividual differences were apparent in individual leptin profiles. Periodogram analysis revealed that only a few animals exhibited 24-h periodicities; the presence of a significant 24-h periodicity was more common in hamsters exposed to short days. Photoperiod-associated differences in the 24-hour profile of leptin secretion may be the result of photoperiod-associated changes in feeding behavior or metabolism. A full understanding of the regulation of leptin secretion in multiple time domains may enhance our understanding of the function of leptin.