The expression of vgf gene, first isolated as a gene induced by nerve growth factor in PC12 cells, was investigated in neurons of the suprachiasmatic nucleus (SCN) by in situ hybridization. In the rat forebrain, the vgf mRNA was found most densely in the SCN. Neurons which express vgf mRNA were found both in the dorsomedial and ventrolateral subdivisions. Double-labeling of vgf in situ hybridization and peptide immunocytochemistry demonstrated that vgf mRNA was expressed in most vasopressin- and neurophysin-immunoreactive neurons in the dorsomedial part and in vasoactive intestinal peptide (VIP)- and peptide histidine isoleucine amide (PHI)-immunoreactive neurons in the ventrolateral part. These findings suggest that vgf is a highly expressed gene in both vasopressin/neurophysin neurons and VIP/PHI neurons which were speculated to be involved in the generation and entrainment of circadian rhythm.