The suprachiasmatic nucleus (SCN) generates and controls the circadian rhythms in mammals including the rhythm of locomotor activity. Leptin is a hormone secreted by adipose tissue that informs the brain about the fat stores. SCN neurons express leptin receptors. Here we investigated the effects of 3 different leptin administrations on the locomotor activity of the Syrian hamsters maintained in constant darkness. Animals were intraperitoneally (ip) injected (4 mu g/kg), subcutaneously (sc) infused (4 mu g/kg), or intra-SCN infused (0.4 mu g/kg) with leptin for 3 days at circadian time 10 whereas the controls received saline (0.9% NaCl) at the same time in order to eliminate stress factors. Our results demonstrate that the locomotor activity of the hamsters can be phase advanced by the external leptin administrations. Leptin affected the level of phase-shifts in an administration method-dependent manner. The biggest phase advance was observed in intra-SCN infusion (P = 0.001), and the smallest was in the ip injection (P = 0.041) group. The wheel-turn amounts did not change significantly in the groups before and after the leptin administrations (P = 0.233); however, the period lengths increased (P = 0.011) significantly after leptin administrations. These results suggest for the first time that in vivo leptin administrations may change the rhythm of locomotor activity in adult male Syrian hamsters.