OBJECTIVE Secretin has been shown to mediate feedback control of pancreatic secretion of fluid and bicarbonate in rats, guinea pigs, and dogs. However, little is known about secretin in the feedback regulation in humans. We investigated the roles of secretin, cholecystokinin, neurotensin, and pancreatic polypeptide on feedback regulation of pancreatic secretion in 10 human volunteers. METHODS A 5-lumen tube was positioned in the proximal jejunum of fasting subjects under fluoroscopy so that gastric juice via lumen 1 and duodenal contents via lumen 3 were collected separately in 15-minute samples while polyethylene glycol solution was infused into duodenum via lumen 2. An acidified (pH 2.0) 4.25% amino acid mixed with phenol red was infused into proximal jejunum via lumen 4, which was alternated with NaHCO3 (control solution) or trypsin (test solution) via lumen 5 intermittently every 15 minutes during separate test periods. RESULTS Infusion of control solution significantly increased both bicarbonate (total change [delta], 7799 +/- 1400 mumol/h) and chymotrypsin (delta 5500 +/- 762 mumol/h) outputs and levels of all four plasma hormones. The test solution significantly inhibited both bicarbonate (delta 2999 +/- 700 mumol/h; P < 0.01) and chymotrypsin output (delta 1000 +/- 120 U/h, P < 0.01), which coincided with a significant suppression of plasma concentration of secretin and cholecystokinin but not pancreatic peptide and neurotensin. CONCLUSIONS A negative-feedback regulation of pancreatic secretion of bicarbonate and enzyme occurs in humans and is mediated via both secretin and cholecystokinin.