The purpose of this study was to evaluate an effect of reflux of the pancreatic juice into the bile duct on the hepato-biliary system. Forty mongrel dogs including 35 adults and 5 puppies were used and divided into two groups according to the procedures to induce the reflux Group I (15 adults and 2 puppies): the main pancreatic duct was anastomosed to the fundus of the gallbladder. Group II (20 adults and 3 puppies): the duodenal pouch including the opening of the main pancreatic duct was anastomosed to the fundus of the gallbladder. In this model the pancreatic juice flowed into the bile duct after activation within the duodenum. Amylase and phospholipase A2 activities in the bile increased in both groups. Activation of phospholipase A2 was more dominant in Group II than in Group I. Pure pancreatic phospholipase A2 was activated only if the pancreatic juice was mixed with bile and its activation became more dominant when it entered into the duodenum. In both groups, the bile duct was dilated, and histology showed destruction of the mucosa and elastic fibers with inflammatory cell infiltration. These changes were especially severer in Group II with reflux of the pancreatic enzymes activated within the duodenum. In the liver, mild fibrosis and inflammatory cell infiltration in the portal area were observed. On the pathogenesis of hepato-biliary changes due to reflux of the pancreatic juice into the bile duct, phospholipase A2 is one of the most important factors among pancreatic enzymes.