The role of oxygen-derived free radicals in the pathogenesis of acute pancreatitis has been investigated in a series of studies using an ex vivo, perfused canine pancreas preparation. Three models of experimental acute pancreatitis have been developed in this preparation: ischemic pancreatitis, gallstone pancreatitis, and alcohol-induced pancreatitis. In each model, the pancreas becomes edematous, gains weight, and the perfusate develops hyperamylasemia during the 4 hour period of perfusion. Pretreatment with the free radical scavengers superoxide dismutase and catalase significantly ameliorates these manifestations of pancreatic injury in each of the three models. The source of the free radical generation was investigated by pretreating the preparation with allopurinol, a quite specific inhibitor of xanthine oxidase. In each of the three models, this also significantly ameliorated the injury process. These experiments demonstrate that oxygen-derived free radicals, generated by activated xanthine oxidase, appear to play a central role in the pathogenesis of acute pancreatitis in these models. These findings shed light on the fundamental pathophysiology of this disease and may provide the basis for more effective therapy in the future.