The following changes were found by either light or electron microscopic observation of the pancreas in spontaneously developed chronic pancreatitis models (WBN/Kob rats, spontaneously hypertensive rats, and rats with common bile-pancreatic duct stones) and in experimental models of chronic pancreatitis (alcoholic pancreatitis, ischemic pancreatitis, and obstructive pancreatitis): 1) the units of lobules, which were constituted by acinar cell deletion, ductular proliferation, and fibrosis; and 2) tortuous or helical ductal channels of pancreatic ducts with periductal fibrosis, which had many crater-like depressions and very long cilia in their inner surface. These are considered to be the results of obstructive pancreatitis, which are caused by the reactions of defensive factors against the increase of pancreatic duct pressure, including the apoptosis of acinar cells, the hyperplasia and hypertrophy of duct cells, a tighter junctional complex of duct cells, and periductal fibrosis.