Results of studies on the effects of exogenous galanin on islet cell secretion are controversial. Until recently, only pig galanin has been available, and structural dissimilarities among the galanin molecules of different species might have contributed to discrepancies among the study results. Thus, we investigated the influence of synthetic rat galanin (50 nM) on unstimulated insulin, glucagon, and somatostatin release and on the responses of these hormones to arginine (10 mM), glucose (16.6 mM), and vasoactive intestinal polypeptide (VIP; 1 nM) in a homologous animal model, the perfused rat pancreas. In addition, the effect of an equimolar concentration of pig galanin on arginine-induced islet cell secretion was examined. Infusion of rat galanin reduced unstimulated insulin release (approximately 60%, P less than 0.01) and the insulin responses to arginine (approximately 30%, P less than 0.025), glucose (100%, P less than 0.01), and VIP (approximately 80%, P less than 0.025). Galanin also inhibited unstimulated somatostatin secretion (approximately 15%, P less than 0.05) and virtually abolished the somatostatin output evoked by arginine, glucose, and VIP. Conversely, rat galanin increased unstimulated glucagon output (approximately 20%, P less than 0.05), potentiated the glucagon response to arginine (approximately 50%, P less than 0.05) and VIP (approximately 90%, P less than 0.05), and counteracted the suppressor effect of glucose on alpha-cell secretion. Pig galanin inhibited the insulin output elicited by arginine (approximately 45%, P less than 0.05) but did not affect the somatostatin and glucagon responses to the aminogenic stimulus. In conclusion, the opposite effects of galanin on insulin and glucagon secretion favor the concept of galanin as a diabetogenic agent. Galanin also behaves as a potent inhibitor of somatostatin release. Finally, the importance of using homologous galanin to study the biological activity of this peptide must be emphasized.