A peptide has been extracted from porcine pancreas which is capable to stimulate gastric acid secretion in the rat. This peptide consists of 13 aminoacids, it's molecular weight is about 1300. In comparison with antral gastrin there are several biochemical differences: GSP (gastrosecretagogue pancreatic polypeptide) reacts differently in gel filtration, ion exchange chromatography and electrophoreses and has a different aminoacid composition. The 4 amino acids of the C-terminal tetrapeptide of gastrin are not all present. GSP does not react with gastrin antibodies in radioimmunoassay. GSP may be a new gastrointestinal hormone by means of which the pancreas takes part in the regulation of gastric acid secretion.