It has been reported that the xanthine oxidase inhibitor, allopurinol, has a protective effect on ischemia - reperfusion injury, but the precise mechanism of its action is still unclear. Therefore, in the present study the mechanisms of the myocardial protection of allopurinol were evaluated in isolated perfused rat hearts. Allopurinol significantly inhibited myocardial xanthine oxidase activity, and improved left ventricular dysfunction after ischemia - reperfusion. In addition, the lactate dehydrogenase content in the coronary effluent obtained after reperfusion was significantly decreased. ATP, ADP, AMP and IMP significantly decreased, whereas inosine, hypoxanthine and xanthine significantly increased after ischemia in both the control and allopurinol groups. The concentration of xanthine was significantly decreased after ischemia - reperfusion in the allopurinol group; however, allopurinol did not affect the other purine metabolites. To evaluate the accumulation of oxidative stress, thiobarbituric acid reactive substances (TBARS) production in myocardial tissue was measured and allopurinol significantly decreased TBARS formation after ischemia - reperfusion. Finally, myocardial hydroxyl radicals were directly measured by electron spin resonance spectroscopy with the nitroxide radical 4-hydroxy-2, 2,6,6-tetramethyl-piperidine-N-oxyl. Hydroxyl radicals significantly increased immediately after reperfusion, but were significantly decreased in the allopurinol group. In conclusion, allopurinol reduced myocardial injury after ischemia-reperfusion by suppressing oxidative stress, but not by salvage of ATP. These findings may lead to the development of new therapeutic strategies for myocardial ischemia - reperfusion injury.