This work investigates the pancreatico-trophic action of bombesin, a peptide stimulating pancreatic secretion in vivo and in vitro and examines whether this effect is altered by CCK or secretin. Rats were injected three times daily for 5 days either with saline, bombesin (1 or 10 μg/kg), CCK (20 I.D.U. kg), secretin (20 C.U./kg) or a combination of bombesin and CCK or secretin. Bombesin alone induced growth of the pancreas beginning with the dose of 10 μg/kg. This growth was characterized by an increase of pancreatic weight, its protein, RNA and enzyme content, but not of its DNA content. The ratios of the three former parameters to DNA increased, suggesting cellular hypertrophy. Cholecystokinin alone exhibited a similar trophic action on the pancreas as bombesin. Secretin alone had no effect on pancreatic size and composition. Cholecystokinin and secretin did not modify the pancreatico-trophic response of bombesin. These findings suggest that bombesin, administered for 5 days in the rat, induces pancreatic growth due to cellular hypertrophy. Cholecystokinin has the same effect. However the mode of action, at the cellular level, of bombesin appears to be different from that of cholecystokinin.