The mechanism by which intraluminal proteases inhibit pancreatic secretion and CCK release was investigated in conscious rats. We hypothesized that the stimulation of pancreatic secretion and CCK release that occurs in the absence of luminal trypsin is caused by a trypsin-sensitive, cholecystokinin (CCK)-releasing peptide that is tonically secreted intraluminally by the small intestine. We tested whether rapid saline perfusion of the lumen of the proximal intestine in rats with jejunostomies would wash out the putative peptide, thereby inhibiting the spontaneous pancreatic secretion caused by diverting bile and pancreatic juice from the intestine. Rats were prepared with cannulas draining bile and pancreatic juice, a duodenal cannula and a jejunostomy 10-12 cm from the ligament of Treitz. During diversion of bile and pancreatic juice to the exterior, the proximal intestine was perfused with phosphate-buffered saline at 3 ml/min via the duodenal cannula and the intestinal washes collected from the jejunostomy outlet. Rapid intestinal perfusion significantly inhibited pancreatic protein and fluid secretion stimulated by diversion of bile and pancreatic juice to the exterior. Reinfusion of the concentrated intestinal washes prevented the "washout" inhibition. The active factor in the intestinal washes was heat stable and trypsin sensitive. Rapid washout perfusion of isolated jejunal loops in Thiry-Vella fistula rats reduced plasma CCK from 20.4 +/- 3.6 to 10.4 +/- 1.8 pM, and reinfusion of the washes into the loop returned plasma CCK to 17.1 +/- 3.8 pM. The results support the hypothesis that a trypsin-sensitive, CCK-releasing peptide in intestinal secretions mediates feedback regulation of pancreatic secretion in rats.