The effects of xanthine-xanthine oxidase reaction on brain microsomal Na⁺-K⁺-ATPase activity were studied to see possible involvement of oxygen free radicals in pathologic change occurring in ischemic state of CNS accompanied by cerebral vascular occlusion or impact injury. When microsomal fraction was incubated with xanthine ana xanthine oxidase, Na⁺-K⁺-ATPase activity of the fraction was markedly inactivated (80% inactivation) whereas btssl Mg⁺⁺-ATPase was much less sensitive (less than 10% inactivation) compared to that of Na⁺-K⁺-ATPase. The inactivation was observed only in the presence of both xanthine and xanthine oxidase, not either of them alone, and the extent of inactivation was dependent on the concentration of xanthine. In an attempt to determine which of the oxygen species was responsible for the inactivation, the ability of various scavengers to overcome the inactivation was tested. Superoxide dismutase, catalase and 1,4-diazabicyclo(2,2,2)octane were shown to reverse the inactivation of the ATPase in dose-dependent manner. In contrast, mannitol as well as other OH⋅quenchers were ineffective in limiting oxygen radical-induced inactivation. Thus OO^-₂⋅, H₂O₂ and ¹O₂ were implicated to be mediators involved in the inactivation. Since oxygen radicals are suspected as being a cause of the peroxidative damaging process in train ischemia, the ATPase inactivation by oxygen radicals may be a possible contributing factor which gives rise to functional derangement of nerve cells observed in the pathologic process.