The important role of oxygen radicals in acute experimental pancreatitis was demonstrated by study of the changes in the antioxidant system in the blood, liver, kidney and pancreas of rats after the administration of a large quantity of L-arginine (L-Arg). The changes in lipid peroxidation and in reduced and oxidized glutathione were followed as well as the activities of peroxide-decomposing enzymes (glutathione peroxidase and catalase) and H2O2-producing superoxide dismutases. The results demonstrated that acute pancreatitis and "oxidative stress" develop rapidly after L-Arg treatment. "Oxidative stress" symptoms are expressed 24 hours after the final treatment. Slow restitution of the studied antioxidant system can be demonstrated as early as after 48 hours.