This study was undertaken to establish if a correlation exists between chronic elevated pancreatic secretion and growth of the pancreas. Rats provided with jugular, pancreatic, biliary, duodenal, or ileal cannulas were fed throughout the experiment with a liquid diet continuously infused into the duodenum. Four days after surgery, control rats and those infused with cerulein (CE) 0.45 microgram/kg/h had their pancreatic juice returned into the duodenum. Two other groups had their pancreatic juice either totally diverted outside (DO) or returned into the ileum (DI). In all groups, bile was returned into the duodenum. Pancreatic juice was collected every 4 h for 4 days with volume and protein determined. After 4 days, rats were killed and their pancreata were evaluated for weight and contents of DNA, RNA, protein, amylase, and chymotrypsinogen. The average volumes/4 h were significantly increased by 259, 241, and 270% in DO, DI, and CE rats, respectively. Protein output remained at control levels in DO rats, whereas increases of 200 and 90% above control values were observed in DI and CE rats, respectively, during the last periods of collection. Constant drainage of pancreatic juice outside (DO) had no effect on pancreatic growth; on the contrary, its reinfusion into the ileum and constant cerulein infusion were associated with impressive growth of the pancreas, with cerulein being the most potent stimulus. In conclusion these data support the hypothesis that increased protein output is associated with pancreatic growth, a phenomenon mediated by endogenous cholecystokinin.