Reactive oxygen species have been involved in pathogenesis of ischemia/reperfusion injury of the liver. It is hypothesized that the phagocytic cells mainly generate the free radicals in reperfusion injury. The aim of this study was to determine whether superoxide radicals are generated in cytoplasm following warm ischemia/reperfusion of murine liver. Transgenic mice overexpressing CuZn superoxide dismutase and normal mice underwent partial-liver ischemia and reperfusion for this purpose. Partial ischemia was performed by clamping hepatic vessels of left and median lobes for 15 minutes. We measured alanine aminotransferase (ALT), hyaluronic acid and phosphatidylcholine hydroperoxide (PCOOH) which was a primary peroxidative product of main component of cell membrane. Forty-five minutes after reperfusion, plasma ALT and PCOOH concentration in the liver were significantly decreased in transgenic mice compared to normal mice. These results suggest that superoxide radicals produced in cytoplasm play a definitive role in the tissue injury after reperfusion of the liver.