The beneficial effects of HBO on ischemic tissue are systemic and not local. (37) The systemic mechanism by which HBO decreases neutrophil adherence during reperfusion is not yet known. A direct action of HBO at neutrophil-endothelial interface is one possibility. Kaelin (15) found a significant crease in the activity of the free radical scavenger superoxide dismutase (SOD) in rats with ischemic skin flaps treated by HBO. If superoxide is playing a direct or indirect role in neutrophil endothelial adhesion, then increased SOD activity may decrease this adherence. Thom (32) studied lipid peroxidation in vitro and demonstrated that hyperoxia alters the free radical pathway in favor of hydroperoxyl radical formation. These quenching radicals react with lipid radicals to form nonradical products. This termination reaction antagonizes and