In pinealectomized rats, melatonin receptor density in the suprachiasmatic nuclei was recently shown to be strongly decreased during the dark period. Furthermore, a one-hour long light pulse is able to reverse this dark-induced decrease. We demonstrate here that this effect of a light-pulse during night time is blocked by prior application of the NMDA receptor antagonist MK-801. These results show that the light effect on melatonin receptor density in the SCN is mediated by activation of NMDA-type receptors.