The two forms of pituitary adenylate cyclase-activating polypeptide, PACAP27 and PACAP38, are novel members of the vasoactive intestinal peptide (VIP)/secretin/glucagon family of peptides. PACAP receptors that are positively coupled to adenylate cyclase and phospholipase C have been recently identified. We examined the expression of PACAP receptors in the rat cortex, hippocampus, cerebellum and hypothalamus during postnatal development. Functional studies revealed PACAP stimulation of cAMP formation in all the brain areas examined and [3H]inositol monophosphate ([3H]insP) accumulation only in the cerebellum and hypothalamus. Throughout development, the efficacy of PACAP in stimulating cAMP formation slightly increased in the cortex and hypothalamus and decreased in the hippocampus and cerebellum; PACAP stimulation of [3H]lnsP formation decreased in the cerebellum and remained steady in the hypothalamus. The effects of PACAP27 and PACAP38 on cAMP levels and inositol phospholipid hydrolysis were dose-dependent between 1 and 100 nM. In the same brain areas, treatment with VIP increased cAMP formation at doses greater than 100 nM and failed to affect [3H]lnsP content, thus suggesting the existence of type-l PACAP receptors. The reverse transcription polymerase chain reaction (RT-PCR) was used to analyse the mRNA expression of type-l PACAP receptor splice variants. PACAP receptor gene expression in the central nervous system was regulated in a developmental- and tissue-specific manner. The PACAP-R transcript was detected in all the brain areas examined whereas PACAP-R-hop mRNA occurred only in the cerebellum and hypothalamus. The different expression profiles and functional properties of PACAP receptors in the developing rat brain suggest an involvement of PACAP in histogenesis, maturation and neurotransmission.