The role of free radicals in the development of pancreatitis was evaluated by measuring the level of activities of xanthine oxidase (XOD), lipid peroxide (LPO) and superoxide dismutase (SOD). Acute pancreatitis was induced in female mice fed a choline-deficient meal containing 0.5 % DL-ethionine (CDE meal). Acute pancreatitis was confirmed by the changes in serum amylase level and other typical features observed microscopically 24 h after the meal was taken. Activity of XOD was elevated significantly (p < 0.05) from the baseline of 1.13 ± 0.19 U/g tissue to 2.34 ± 0.46, 2.59 ± 0.33 and 3.46 ± 0.70 U/g tissue, 8, 12 and 24 h, respectively, after the CDE meal. The LPO level was also increased from an undetectable amount to 1.10 ± 0.47 nmol/ml (p < 0.05), 1.03 ± 0.18 (p < 0.01) at 6 and 8 h, respectively, and then returned to an undetectable amount at 12 h. The peak level of LPO was shown at 24 h, 1.76 ± 0.40 nmol/ml (p < 0.01) and gradually decreased until 48 h, 1.17 ± 0.37 nmol/ml (p < 0.01) after the CDE meal. Changes of LPO took a biphasic pattern. SOD was decreased significantly from 47.1 ± 3.4 mU/g tissue to 30.7 ± 2.5, 24.8 ± 1.7 and 20.6 ± 1.1 mU/g tissue at 8 (p < 0.01), 12 (p < 0.01), and 24 (p < 0.01) h, respectively. These results indicate that oxygen-derived free radicals play an important role in the development of acute pancreatitis and that the imbalance of the offense system represented by XOD and LPO and the defense system reflected by SOD in the tissue might be an important cause of tissue damage induced by oxygen-derived fr