Abstract.It has recently been demonstrated that the infusion of a high caloric load (3.3 kcal min‐1≙ 14.0 kJ min‐1) into human upper jejunum inhibited pancreatic enzyme and bile salt secretion. The aim of the present study was to investigate whether this phenomenon was mediated by gastrointestinal hormones which interfere with pancreatic secretion. In six healthy volunteers, jejunal infusion of 1.3 kcal min‐1(5.5 kJ min‐1) did not modify secretion of lipase and chymotrypsin to any significant extent compared with saline infusion, but the rate of 3.3 kcal min‐1(14.0 kJ min‐1) resulted in an inhibition. Somatostatin and pancreatic polypeptide, which are known to inhibit exocrine pancreatic secretion, remained unchanged during jejunal nutrient infusion. The inhibition of pancreatic enzyme secretion was observed in temporal relationship with an increase of the stimulators of pancreatic exocrine secretion such as secretin, neurotensin, and CCK. The existence of an hitherto undetined inhibitor and a feedback mechanism