In a patient a papilla Vateri tumor completely prevented the bile-pancreatic flow into the intestine although the pancreatic juice was secreted into the bile duct via a common channel. Consequently, the bile-pancreatic juice was possible to sample via a percutaneous transhepatic cholangiography (PTC) catheter. This made it possible to study the effect of duodenal infusion of different substances on the bile-pancreatic secretion. In repeated experiments a suppression of the secretion was observed by intraduodenal trypsin as well as the patient’s own bile-pancreatic juice. In the presence of bile-pancreatic juice intraduodenal trypsin inhibitor infusion caused a marked stimulation of the secretion. The results are in accordance with the hypothesis that trypsin in the upper part of the intestine exerts a negative feedback regulation of the pancreatic secretion in ma