Pituitary adenylate cyclase activating polypeptide (PACAP) specific radioimmunoassay systems have been developed and the distribution of PACAPs in guinea pig tissues has been studied. The antibody against PACAP38 was characterized, using synthetic peptide fragments. It was shown to recognize the C-terminal portion with the C-terminal amide group and no cross-reaction was observed with vasoactive intestinal polypeptide (VIP), which has a high homology with PACAP27 and the N-terminal 28 amino acid residues of PACAP38. The antibody against PACAP27 was specific to PACAP27. With the two PACAPs (PACAP38 and PACAP27) specific RIA systems, high concentrations of PACAP38- and PACAP27-like immunoreactivity (LI) were observed in the brain of guinea pigs, especially in the diencephalon (mostly hypothalamus). In all tissues PACAP38-LI was higher than that of PACAP27-LI. Reverse-phase HPLC showed that PACAP38- and PACAP27-LI of tissue extracts were superimposed at the elution position of those of the synthetic peptides, respectively.