This study was undertaken to investigate serially the development, progression, and healing of acute pancreatitis which was induced in rats by the closed duodenal loop technique. Edematous changes appeared within 2 to 4 h. Pancreatic blood flow increased at 1 h and tended to decrease after 3 h. After injection of fesin into the loop, fesin appeared in the periacinar space and acinar cells after 4 h. Electron microscopy showed that partial destruction of the plasma membrane occurred and serum amylase increased considerably at the same time, suggesting that the initial inflammatory changes caused the release of pancreatic enzymes at approximately 4 h. After elimination of the causal factors, edematous pancreatitis healed completely but hemorrhagic acute pancreatitis was not completely healed 6 months later.