A hypothesis that superoxides produced by NADPH oxidase promote early phase injury of reoxygenated liver was tested. Rats that were given a inhibitor of NADPH oxidase, apocynin, were subjected to 45 minutes warm hepatic ischemia and 60 minutes reperfusion. Pretreatment of the rats with apocynin significantly reduced elevation of serum alanin aminotransferase and concentration of hyaluronic acid. It also improved deteriorated energy charge of the liver and gadolinium chloride administration facilitated this effect. It is likely that superoxides generated by NADPH oxidase of hepatic macrophages participate in initiation of early phase injury after hepatic warm ischemia and apocynin is a potential therapeutic tool for prevention of reperfurion injury.