Much evidence suggests that free radicals and active oxygen species derived from molecular oxygen (superoxide, hydrogen peroxide, and hydroxyl radical) contribute to the tissue injury which accompanies myocardial ischemia and reperfusion. Three possible sources have been identified for the production of active oxygen species: the enzyme xanthine oxidase; the activated polymorphonuclear leukocyte; the disrupted mitochondrial electron transport system. These sources may be mutually interactive. Once triggered, they may lead to the loss of antioxidant enzymes and to the release of iron, both of which are exacerbatory events.