In the Syrian hamster a serotonergic (5-HTergic) stimulation during daytime acts on the circadian timing system by inducing behavioral phase advances and by decreasing Per1 and Per2 (Period) mRNA levels in the suprachiasmatic nuclei, containing the main circadian clock in mammals. The present study was conducted in Syrian hamsters, housed in constant darkness, to investigate the interactions between light or melatonin with serotonergic stimulation in terms of phase resetting and clock gene expression. Both light exposure and systemic administration of melatonin prior to the injection of a 5-HT(1A/7) receptor agonist, 8-OH-DPAT, in the middle of the day blocked behavioral phase advances. In contrast, neither light nor melatonin treatment during daytime prevented serotonergic-induced down-regulation of Per1 and/or Per2 mRNA levels in the suprachiasmatic nuclei. Taken together, the results show that interactions between afferent cues to the suprachiasmatic nuclei differentially modulate phase adjustment and clock gene expression during daytime.