In the current study we evaluated effluent blood from extremities of human patients undergoing reconstructive surgical treatment which is routinely accompanied by upper extremity exsanguination and application of a tourniquet. Following tourniquet release (reperfusion), there were immediate increases in the plasma levels of xanthine oxidase activity, uric acid, and histamine. Xanthine dehydrogenase activity was not detectable. Plasma also contained products consistent with the formation of oxygen-derived free radicals, namely hemoglobin and fluorescent compounds. Our data indicate in humans that ischemia-reperfusion events are associated with the appearance of xanthine oxidase activity and its products in the plasma effluent.