To study the effects of chronic bombesin on pancreatic growth and secretion, rats were injected subcutaneously, 3 times daily for 4 days with either saline or bombesin (10 micrograms/kg). Bombesin significantly increased the pancreatic weight and content in protein and RNA but not in DNA. The ratios of the three former parameters to DNA increased, suggesting cellular hypertrophy. The pancreatic content in enzymes was also elevated, especially for chymotrypsin and to a lesser degree for amylase. However, the volume of pancreatic secretion and the output of enzymes in response to CCK under a continuous infusion of secretin remained unchanged. The in vitro secretory response to caerulein and bombesin was reduced for amylase and lipase. It is concluded that chronic bombesin exerts a trophic action on the rat pancreas but decreases the sensitivity of each cell to hormonal stimulation.