An ideal canine model of anomalous pancreaticobiliary ductal union, similar to a human anomaly, was devised. Direct anastomosis of the dorsal pancreatic duct and the choledochus was employed (dorsal pancreaticocholedochostomy). The ventral pancreatic duct was not manipulated. Cylindrical choledochal dilatation results in 13 out of 20 adult dogs and in 4 out of 6 puppies. Dilatation of the intrahepatic biliary tree and thickening of the choledochal wall in a hepatic direction were observed in high ratio among adult dogs. Puppies had less dilatation. The activation of pancreatic enzymes was studied in bile containing pancreatic juice. The proteolytic enzymes, trypsin and elastase were proven to be activated in the bile without the presence of enterokinase in the pancreaticocholedochostomy model. This would be the cause of the ill effects of refluxed pancreatic juice into bile in anomalous pancreaticobiliary ductal union.