A B S T R A C T Bombesin (a tetradecapeptide), the C-terminal nonapeptide of bombesin (bombesin-NP), and litorin (a parent nonapeptide), each stimulated amylase secretion from rat pancreatic fragments. These respoiises were not affected by atropine. The concentrations that produced half-maximal stimulation of secretion were 0.25 nM for bombesin, 0.30 nM for bombesin-NP, and 0.70 nM for litorin, as compared to 0.12 nM for caerulein and 0.80 AM for the cholinergic agent carbamylcholine. When used at maximal concentrations, bombesin, bombesin-NP, and litorin showed no action on cyclic AMP levels in the presence of 5 mM theophylline. By contrast, caerulein and secretin increased cyclic AMP levels by 27 and 208%, respectively. Bombesin, bombesin-NP, and litorin did not activate adenylate cyclase in a purified pancreatic plasma membrane preparation, whereas caerulein and secretin increased this activity 20 and 16-times, respectively.Calcium efflux was estimated in isolated acinar cells from the rat pancreas. The concentration of bombesin, bombesin-NP, litorin, caerulein, the C-terminal octapeptide of cholecystokinin-pancreozymin, and carbamylcholine that produced half-maximal stimulation of calcium efflux were 0.30 nM, 0.50 nM, 2.00 nM, 0.80 nM, 0.20 nM, and 3.00 uM, respectively. Maximal stimulation of calcium efflux by bombesin or C-terminal octapeptide of cholecystokinin-pancreozymin lasted only 10 min. In