Acute pancreatitis may be initiated in the ex vivo, perfused canine pancreas preparation by a variety of stimuli. These include oleic acid infusion (FFA), partial duct obstruction with secretin stimulation (POSS), and a 2-hour period of ischemia (ISCH). In each model, pancreatitis is characterized by weight gain, edema, and hyperamylasemia. Oxygen-derived free radicals such as superoxide, hydrogen peroxide, and the hydroxyl radical are highly reactive toxic substances that are normally produced in small amounts during oxidative metabolism. Ordinarily, these substances are detoxified by endogenous intracellular enzymes called free radical scavengers (FRS), such as superoxide dismutase (SOD) and catalase (CAT). These studies were undertaken to evaluate the possible role of oxygen-derived free radicals in the initiation of acute pancreatitis in the isolated canine model. All preparations were perfused for 4 hours with autologous blood. Controls (N = 6): these glands remained normal in appearance, gained minimal weight (6 +/- 1 g), and serum amylase remained normal (less than 1000 u/dl). FFA pancreatitis, FFA alone (N = 6): these glands became edematous, gained weight (113.5 +/- 27.0 g), and developed hyperamylasemia (2087 +/- 387 u/dl). FFA + FRS (N = 6), SOD (50 mg) and CAT (50 mg) were added to the perfusate at time zero: these glands became only minimally edematous, gained less weight (31.8 +/- 10.1 g, p less than 0.05), and amylase remained normal (p less than 0.05). POSS pancreatitis, POSS alone (N = 8): these glands became edematous, gained weight (38.6 +/- 4.6 g), and developed marked hyperamylasemia (9522 +/- 3226 u/dl). POSS + FRS (N = 6): these glands did not develop edema, gained less weight (15.1 +/- 2.6 g, p less than 0.05), and serum amylase only increased to 1815 +/- 343 u/dl, (p less than 0.05). ISCH pancreatitis, ISCH alone (N = 6): these glands became edematous, gained weight (75.8 +/- 25 g), and developed hyperamylasemia (1679 +/- 439 u/dl). ISCH + FRS (N = 6): these glands did not develop edema, gained only 18.3 +/- 9.0 g (p less than 0.005), and serum amylase remained normal (p less than 0.05). These studies demonstrate that, in this canine preparation, acute pancreatitis is significantly ameliorated by oxygen-free radical scavengers. Since this was true whether the pancreatitis was produced by FFA infusion, POSS, or ischemia, it suggests that oxygen-derived free radicals may mediate a common essential step in the pathogenesis of all forms of pancreatitis.