The effects of intracerebroventricular (i.c.v.) administration of pituitary adenylate cyclase-activating polypeptide (PACAP) on the expression of c-fos gene as well as heteronuclear (hn) RNA for arginine vasopressin (AVP) in paraventricular (PVN) and supraoptic nuclei (SON) of rats were investigated by immunohistochemistry for c-fos protein (Fos) and in situ hybridization histochemistry for c-fos mRNA and AVP hnRNA. The i.c.v. administration of PACAP (200 pmol/rat) caused a marked induction of Fos-like immunoreactivity (LI) in PVN and SON. The nuclear Fos-LI existed in AVP-LI containing cells in the PVN and SON. The expression of the c-fos gene in the PVN and SON was increased in a dose-related manner 30 min after i.c.v. administration of PACAP. PACAP-induced expression of the c-fos gene was significantly reduced by pretreatment with a PACAP receptor antagonist, PACAP-(6-38)-NH<sub>2</sub>. In addition, Fos-LI and the expression of the c-fos gene were also observed in the periventricular region of the third ventricle after i.c.v. administration of PACAP. The induction of c-fos gene expression in the PVN and SON reached a maximum 30 min after PACAP administration. The expression of c-fos gene in the PVN and SON induced by i.c.v. administration of vasoactive intestinal peptide (200 pmol/rat) was weaker than that induced by PACAP. The hnRNA for AVP in the PVN and SON was significantly increased 30 min after i.c.v. administration of PACAP (200 pmol/rat). Our results suggest that PACAP activates PVN and SON neurons via PACAP receptors and, in parallel, transcription of the AVP gene in the PVN and SON.