Porcine GIP (gastric inhibitory polypeptide) potentiates glucose-induced insulin secretion under a variety of experimental conditions. Recently GIP was isolated also from bovine intestine, and found to differ from porcine GIP by having isoleucine instead of lysine in position 37. We have compared the effects of porcine GIP to that of bovine GIP and bovine GIP(1-39) on glucose-induced insulin secretion from the perfused rat pancreas. We found that porcine GIP, bovine GIP, and bovine GIP(1-39) all strongly potentiated both first and second phases of glucose-induced insulin secretion (glucose concentration 6.7 mM; polypeptide concentration 1 nM). There was no significant difference between the polypeptides with regard to the potency to potentiate glucose-induced insulin secretion. We conclude that bovine GIP, as porcine GIP, potentiates glucose-induced insulin secretion, and that the insulinotropic activity of GIP is not confined to the last three amino acids at the C-terminal end.