Acute pancreatitis was induced by ligating the opossum common biliopancreatic duct immediately proximal to its entry into the duodenum, and macroscopic as well as microscopic changes were evaluated during the subsequent 24 hours. Transient pancreatic edema and progressive hyperamylasemia were noted within 6 hours of pancreatic and bile duct ligation. Light microscopic evidence of pancreatic injury including acinar cell necrosis, hemorrhage, fat necrosis, and inflammatory cell infiltration was noted within 12 hours of duct obstruction. Electron microscopic changes included massive dilatation of the rough endoplasmic reticulum and disruption of the apical plasmalemma of acinar cells during the initial 3 hours. These observations indicate that pancreatic and bile duct ligation in the opossum results in the rapid (less than 24 hours) appearance of changes consistent with acute hemorrhagic and necrotizing pancreatitis and that the initial lesion in this model of experimental pancreatitis involves acinar cells.