The influence of biliary diversion upon gastric acid secretion and the serum gastrin level was investigated in dogs with Heidenhain pouch. Biliary diversion was carried out by the following operative procedures: 1) External biliary fistula (3 dogs), ligation and division of the common bile duct and construction of an external biliary fistula by inserting a cannula into the gallbladder. 2) Roux-en-Y internal biliary fistula (6 dogs), ligation and division of the common bile duct and construction of an internal fistula (cholecystojejunostomy) by anastomosing the gallbladder to the jejunum 10 cm distal to the ligament of Treitz, with restoration of end-to-side jejunojejunostomy. In each dog, a test meal (Dog Food, 300 g) was administered, and the gastric acid secretion and serum gastrin level were examined. In the external biliary fistula, the gastric acid secretion showed no increase after the test meal, while in the Roux-en-Y internal fistula the secretion from the Heidenhain pouch showed a significant increase in 4 of 6 dogs, with an increase rate of 110-330%. In either the external biliary fistula or the Roux-en-Y internal fistula, the serum gastrin level showed no significant change. However, in the dogs with the Roux-en-Y internal fistula allotted for the observation periods of 4 to 5 months, the serum gastrin level at 3 hr after the test meal was significantly higher than that at fasting. Further, in pursuit of the reproducibility of gastric acid secretion with the Roux-en-Y internal biliary fistula in the course between the first and second Roux-en-Y internal fistulae, the gastric acid secretion markedly increased in the cases of the Roux-en-Y internal fistula, without definite changes in the level of serum gastrin. Acute gastric ulcer developed in 2 of 6 dogs with the Roux-en-Y internal biliary fistula, but no fat indigestion was caused. These results suggest that gastric hypersecretion induced by biliary diversion may prove the possibility of presence of gastric secretagogue, different from gastrin, released in the upper jejunum. biliary diversion; internal biliary fistula; gastric acid secretion; serum gastrin level Biliary-intestinal anastomoses are used for reconstruction of the biliary tract. It has been pointed out that the surgical procedure for reconstruction of the biliary tract, particularly choledochoj ej unostomy, is likely to give rise to peptic ulcer. Recently, McArthur and Longmire (1971) reported an increased incidence of peptic ulcer in a relatively large series of patients who had undergone choledochojejunostomy for benign biliary obstruction. However, the possible