This study was undertaken to clarify the issue of whether feedback regulation of pancreatic enzyme secretion occurs in humans. A naturally occurring inhibitor of trypsin and chymotrypsin, the Bowman-Birk inhibitor of soybeans, was used to reduce the activities of these enzymes normally secreted by the pancreas into the duodenum. Pure pancreatic juice was collected by endoscopic retrograde cannulation of the pancreatic duct in a protocol consisting of three periods: period 1 (15 min), collections of juice without return to the duodenum ("washout phase"); period 2 (35 min), intraduodenal infusion of juice to which buffered saline or heat-inactivated Bowman-Birk inhibitor had been added; and period 3 (55 min), intraduodenal infusion of juice in which greater than 90% of the trypsin and chymotrypsin activities had been abolished by treatment with the active inhibitor. Control experiments were included in which untreated juice was infused in period 3 as well as period 2. Enzyme analyses (trypsin, chymotrypsin, elastase, and amylase) of samples of juice collected at 5-min intervals revealed a twofold to threefold increase in the output and concentration of all four enzymes in period 3 compared with period 2. These results thus confirm the existence of feedback control in humans.