Gastrin releasing peptide (GRP) is an intrapancreatic peptide, but its physiological function is unknown. Previously, the peptide has been shown to increase plasma levels of insulin and glucagon in vivo in dogs, but no studies on the possible direct actions on islet hormone secretion from the dog pancreas have been undertaken. Therefore, we examined the effects of a 10-min perfusion of synthetic porcine GRP at four different dose rates over a wide range (0.1-50 nmol l-1) on the islet hormone release from the isolated dog pancreas (n = 5-6 in each group) at 5.5 mM glucose. We found that, at all four concentrations tested, GRP rapidly and markedly stimulated insulin secretion. The stimulation was, however, transient: the increased insulin secretion returned to basal levels within 7-8 min despite the ongoing GRP perfusion for 10 min. In contrast, GRP did not affect the pancreatic secretion of glucagon or somatostatin. We conclude that GRP stimulates insulin secretion by a direct pancreatic action without affecting the secretion of glucagon or somatostatin.