The present study was conducted to determine whether the photic induction of c-fos expression in the rat suprachiasmatic nucleus (SCN) occurs within neurons containing gastrin-releasing peptide (GRP) and/or vasoactive intestinal polypeptide (VIP) because these peptidergic cells are closely associated with retinal projections to the ventrolateral subfield. Using dual immunostaining and thin sectioning techniques, the ventrolateral SCN of light-exposed rats was examined for evidence of individual neurons coexpressing nuclear immunostaining for c-fos proteins (Fos) with cytoplasmic immunoreactivity for GRP or VIP. In all animals, the photic induction of Fos expression in the SCN was mainly confined to cells segregated within the ventrolateral subfield and was evident in approximately 40% of the SCN neurons with cytoplasmic immunoreactivity for GRP. However, neurons coexpressing Fos and GRP comprised only a small fraction of the total number of cells within the ventrolateral SCN exhibiting light-induced Fos immunoreactivity. No sign of Fos expression was detected within VIP-immunoreactive perikarya in the ventrolateral SCN of light-treated rats. These results demonstrate that light induces Fos expression in a number of GRP-containing neurons within the SCN, suggesting that these peptidergic cells may process photic information mediating circadian entrainment.