Because so little is known about the pathological anatomy of acute pancreatitis, it was felt that clarification of the very early stages of this entity might lead to a better understanding of the morphological changes in the pancreas which are seen in the later stage of the disease process. Previous experiments have shown only the nonspecific changes of cell necrosis, hemorrhage, and complete destruction of pancreatic architecture as demonstrated by light microscopy, but now, with the help of the electron microscope, we have been able to demonstrate the early changes in the experimental production of acute pancreatitis, using a modification of the methods of Elliott et al1and Nemir et al.2 Materials and Methods The accessory pancreatic duct was cannulated, extraduodenally, with a fine polyethylene catheter in 30 mongrel dogs, and various combinations of saline, canine bile, and trypsin were allowed to flow into the ductal system for