In mammalian tissues the C-terminal amide structure has been found to occur only in neuroactive or hormonally-active peptides¹. About half known neuropeptide and peptide hormones have this unique chemical feature. Using a chemical detection method², a search for previously unknown peptides that possess the C-terminal amide structure in extracts of brain and intestine was carried out³ and a number of novel neuropeptides and hormonal peptides⁴, designated neuropeptide Y^5,6, PHI⁷, peptide YY⁸, galanin⁹ and neuropeptide K¹⁰ were isolated. We recently performed a similar search in porcine pancreas and found a high concentration of a peptide having a glycine amide at its C-terminus. Here we report the isolation, primary structure and biological activity of this novel peptide. The 49-residue peptide strongly inhibits glucose-induced insulin release from the isolated perfused pancreas and was therefore named pancreastatin. It may be important in the regulation of insulin secretion and in the pathogenesis and treatment of diabetes mellitus.