In the ischemic heart, high-energy phosphates are rapidly broken down. We studied the release of AMP catabolites from the isolated perfused rat heart which was temporarily made ischemic or anoxic. We measured the concentration of purine nucleosides and oxypurines with a novel high-pressure liquid chromatographic technique. The postischemic working heart released adenosine, inosine, hypoxanthine, and also substantial amounts of xanthine. The latter could indicate that xanthine oxidase is present in rat heart. Further evidence for the myocardial occurrence of this enzyme was obtained from experiments with hearts perfused retrogradely with allopurinol, an inhibitor of xanthine oxidase. This drug greatly enhanced the release of hypoxanthine, both during normoxic and anoxic perfusions. We conclude that xanthine oxidase could play an essential role in the myocardial breakdown of AMP catabolites.