In mammals, circadian rhythms generated by the suprachiasmatic nuclei (SCN) are daily synchronized by a light-dark cycle. Photic information is transmitted to the SCN mainly through the direct retinohypothalamic tract, the neurotransmitters involved being excitatory amino acids. It is also commonly accepted that photoperiodic information coming from the retina via the SCN is transduced by the pineal into a nocturnal signal, i.e. melatonin production. Light exposure at night induces (1) an inhibition of melatonin synthesis and (2) an expression of c-fos in numerous cells of SCN. To determine the role of the NMDA receptor in these effects, we treated Syrian hamsters with ip injections of MK-801, a noncompetitive NMDA receptor antagonist. Several subpopulations of light-sensitive cells in the SCN are affected by MK-801. According to previous studies, MK-801 inhibits light-induced Fos immunoreactivity mainly in the most ventral part of the SCN. However, we observed that numerous other cells are still activated by light. When light is applied in the middle of the night, MK-801 pretreatment does not reduce Fos-ir in the dorsal SCN. At the beginning of the night, labeled cells in this part of the nucleus appear even more numerous after MK-801. We also found that MK-801 fails to reduce the light-induced inhibition of melatonin synthesis. Moreover, in control animals, which received no light stimulation, ip injection of MK-801 induces by itself a dose-dependent inhibition of melatonin production.