Acute pancreatitis was induced in 39 dogs by injecting 10 ml of autologous bile into the pancreatic duct system. Of these dogs, 19 received hydrocortisone. As control, used were two normal dogs and six that were injected with 10 ml of normal saline solution into the pancreatic duct. Of all, 14 dogs were examined by electron microscope. All dogs that were injected with bile developed acute pancreatitis and histologically the lesion produced was acute necrotizing pancreatitis. Dogs treated with steroids showed an improved survival and a reduction in the severity of the acinar lesion. Ultrastructurally, pancreatic changes in these animals were mainly in the acinar cells and capillaries. Although ultrastructural lesions were similar in animals treated with steroids, steroid therapy appeared to be associated with an improved structural preservation of the acinar cells. These results suggest that the bile infusion method is reliable in producing acute necrotizing pancreatitis and that steroid administration is associated with an improved survival and reduction of the severity of the pancreatic lesion. The pathogenetic mechanism in this model and the mode of action of steroids were suggested by the ultrastructural study.