Six subjects with a normal endoscopic pancreatogram were investigated after an overnight fast by means of a side-viewing duodenoscope. After cannulation of the main pancreatic duct, juice was collected in five-minute samples for 20 minutes. An iso-osmolar solution of 6 g cattle bile was then infused into the duodenum through a separate catheter attached to the outside of the duodenoscope, and pancreatic juice collected in five-minute samples for another 20 minutes. Blood was frequently drawn from an arm vein through an indwelling catheter for estimation of immunoreactive secretin (IRS) by radioimmunassay. The flow rate of pancreatic juice and outputs of bicarbonate, amylase, and protein increased significantly after intraduodenal infusion of bile. A significant rise in plasma IRS was also found after instillation of bile in the duodenum.