Exposure of rodents to light can induce expression of a number of immediate-early genes, including c-fos, in cells of the suprachiasmatic nucleus (SCN), a dominant pacemaker in the mammalian circadian system. We examined the effects of pre-treatment with the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, on the induction of Fos-like immunoreactivity (Fos-lir) in cells in the hamster SCN. At doses of 3 and 5 mg/kg, MK-801 treatment blocked photic induction of Fos-lir in the rostral SCN and ventrolateral portions of the caudal SCN but failed to block induction of Fos-lir in a discrete region of the dorsolateral SCN. These results suggest that photic induction of Fos-lir in most of the SCN depends on activation of an NMDA-type receptor which is sensitive to MK-801, but that Fos-lir in one portion of the SCN is induced by a mechanism which is not antagonized by MK-801.