The effects of somatostatin-14 (S14) and somatostatin-28 (S28), a novel intestinal peptide containing somatostatin tetradecapeptide in its C-terminal position, on the bombesin-stimulated release of gastrin, insulin, and glucagon were tested. On iv infusion of bombesin, the increase in the level of glucagon was seen to be twice that of gastrin, and the insulin increase was 8 times that of gastrin. Plasma concentrations of somatostatin were not modified. S14 and S28 inhibited the release of gastrin, insulin, and glucagon; insulin release was inhibited most effectively, with glucagon release next, and gastrin release least inhibited. Based on the exogenous dose needed to produce equal effects, S28 was more potent than S14 on a molar basis, but based on the plasma concentrations needed to produce equal effects, S14 and S28 were equipotent.