This study was designed to clarify the mechanism of ischemia-reperfusion injury to skeletal muscle using long-acting polyoxyethylene-modified superoxide dismutase (SOD-POE) in rats. The gastrocnemius muscles of male Lews rats were investigated. Tissue levels of ATP decreased to 20% of nonischemic values after 3 h ischemia and returned to 94% after 1 h reperfusion. In contrast, they decreased to 3.5% after 4 h ischemia and remained at 20% after 1 h reperfusion. Although SOD-POE did not affect the decrease of ATP during ischemia, it improved significantly the recovery of ATP: 28%. Tissue levels of lipid peroxides (LPO) after 3 h ischemia and 1 h reperfusion did not change significantly compared with the nonischemic levels (0.71 ± 0.32 nmol/mg protein, mean ± SD). They showed no increase after 4 h ischemia, but increased explosively after 1 h reperfusion (2.15 ± 0.73 nmol/mg protein). SOD-POE did not affect LPO levels during ischemia but prevented the increase of LPO significantly after reperfusion (0.98 ± 0.25 nmol/mg protein). Xanthine oxidase activity did not increase after 3 h ischemia (22.3 ± 7.0 mU/g) compared with the nonischemic values (17.6 ± 10.0 mU/g). In contrast, it increased 2.5-fold after 4h ischemia (50.1 ± 13.7 mU/g) and remained at a significantly high level after 1 h reperfusion. SOD-POE did not affect xanthine oxidase activity during ischemia and reperfusion. These results suggest that lipid peroxidation by superoxide radicals produced by xanthine oxidase is a contributory factor to ischemia-reperfusion injury to skeletal muscle, and the clinical application of SOD-POE might be ex