Pituitary adenylate cyclase activating polypeptide (PACAP) which belongs to the vasoactive intestinal polypeptide family of regulatory peptides, occurs in two variants, PACAP-27 and PACAP-38, and is thought to be an important messenger in both the central and peripheral nervous system. Three cloned G-protein coupled 7 transmembrane spanning receptors bind PACAP with high affinity, one reacting only weakly with VIP (PACAP type I receptor) the other two binding vasoactive intestinal peptide (VIP) with equally high affinity (VIP type I and II receptors). The PACAP type I receptor displays high affinity for both variants of PACAP. In this study, we have investigated the distribution of PACAP and the PACAP type I receptor in the adrenal medulla of newborn and adult rat. Immunocytochemistry revealed, in the adult rat, a dense network of PACAP-immunoreactive nerve fibers terminating on chromaffin cells. Such fibers were few and weakly immunoreactive in the newborn rat. PACAP-immunoreactive medullary cells could not be detected in the adult rat, whereas in the newborn, occasional cells were seen. By in situ-hybridization we detected PACAP type I receptor mRNA in a majority of the adrenal medullary cells of both newborn and adult rat. Receptor autoradiography using 125I-PACAP-27 as ligand revealed binding-sites with a localization virtually identical to the in situ hybridization signal indicating a functional expression of high-affinity type I PACAP receptors in the adrenal medulla. Additionally, in the adult rat, single or clustered large cells, presumably ganglion cells, contained an even higher abundance of PACAP receptor mRNA as well as binding sites than the surrounding chromaffin cells. Our observations on the distribution of PACAP peptide and PACAP receptors in the adrenal medulla suggest that both chromaffin cells and ganglion cells are PACAP targets. The data thus strengthen earlier observations indicating an important regulatory role of PACAP in catecholamine biosynthesis and release. The presence of both ligand and receptors in newborn rats may indicate a role for PACAP in the development of the adrenal medulla.