Reactive oxygen species that are generated in the ischemic heart upon reperfusion, play a significant role in the pathogenesis of reperfusion injury. Although DNA is a well known target for free radical attack, little attention has been paid to the injury of DNA molecules associated with ischemia and reperfusion. In this study, the formation of 8-hydroxydeoxyguanosine (8-OHDG), a product of hydroxyl radical (OH.)-DNA interaction, was monitored in the post-ischemic myocardium. A simple high performance liquid chromatography (HPLC), with uv detection, detected pmol levels of 8-OHDG in the pre-ischemic heart which increased steadily and progressively as a function of reperfusion time. A similar rise in 8-OHDG was noticed when isolated hearts were perfused with a OH. -generating system. Corroborating with the increased 8-OHDG formation, increased amount of creatine kinase was released from the coronary effluent indicating increased tissue injury. The formation of 8-OHDG was completely blocked when hearts were preperfused with oxygen-free-radical scavenger, 1,3-dimethyl-2-thiourea (DMTU) which also significantly reduced the appearance of CK in the coronary effluent, suggesting that oxidative DNA damage play a role in the pathophysiology of ischemic reperfusion injury.