The role of oxygen-derived free radicals in the pathogenesis of acute pancreatitis was studied in a new model of acute hemorrhagic pancreatitis and cerulein-induced edematous pancreatitis in rats. Hemorrhagic pancreatitis was produced by administering two intraper-itoneal doses of cerulein [40 μg/kg body weight (BW)] at 1-h intervals following water immersion stress applied for 5 h. Edematous pancreatitis was induced by injecting cerulein as described but without water immersion. Five hours after the first injection of cerulein, pancreatic edema and elevation of serum amylase level were more marked in the animals with hemorrhagic than with edematous pancreatitis. Five hours after the first injection of cerulein, marked hemorrhage and venous dilatation were observed only in those with hemorrhagic pancreatitis. Local pancreatic blood flow decreased to ∼60% of control values in the animals with edematous pancreatitis, and to ∼30% of control values in those with hemorrhagic pancreatitis. To evaluate the involvement of oxygen radicals, some rats received three intraperitoneal injections of superoxide dismutase (SOD 10,700 Ulkg BW) and cat-alase (132,000 U/kg BW) beginning 15 min before the first injection of cerulein and repeated at 1-h intervals. No significant effect of free radical scavengers was observed on the edematous pancreatitis. However, in hemorrhagic pancreatitis, treatment with SOD and catalase completely suppressed the hemorrhage and venous dilatation of the pancreas, significantly reduced the pancreatic wet weight and the serum amylase level, and reduced the histologic alterations. However, after treatment with SOD and catalase, no differences were observed in local pancreatic blood flow. These findings indicate that stress produced microvascular damage such as pancreatic hemorrhage following the generation of oxygen radicals, and that these radicals produced severe damage to the pancreas.