Oxygen free radicals are highly toxic end products of the cellular oxidative metabolism that are scavenged by antioxidative systems such as superoxide dismutase and catalase. Previous studies have suggested that oxygen free radicals may play a role in the pathophysiology of acute pancreatitis. The present study was designed to determine the effect of catalase on experimental acute pancreatitis in the rat. Rats were given either saline or enterokinase by the intraductal pancreatic route. Catalase was given intraperitoneally at doses of 80, 160 and 320 mg/kg. Administration of saline resulted in mild acute pancreatitis. In contrast, administration of enterokinase resulted in oedema, haemorrhage and necrosis of the pancreas. Catalase had no effect when given at 80 mg/kg; at 160 mg/kg, an improvement at the ultrastructural level was noted, whereas at 320 mg/kg, a change in serum calcium level was found. Our studies suggest that intraperitoneal administration of catalase slightly improves the outcome of experimental acute pancreatitis in the rat.