Oxygen radicals appear to be involved in the microvascular and parenchymal cell injury associated with various pathologic disorders. Studies indicate that oxygen radicals increase microvascular permeability by creating large leakage sites predominantly in the small venules. The highly reactive hydroxyl radical appears to be responsible for the microvascular alterations associated with oxygen radical production. There is considerable indirect evidence implicating oxygen radicals in the pathogenesis of circulatory shock. The oxygen radicals are probably formed by the enzyme xanthine oxidase when intravascular volume is restored. Similar biochemical processes appear to be involved in reperfusion injury to the kidney and skin. Evidence is also presented that implicates oxygen radicals in the reperfusion injury associated with organ preservation and transplantation.