This study characterizes mitochondria isolated from livers of Sod2(-/+) and Sod2(+/+) mice. A 50% decrease in manganese superoxide dismutase (MnSOD) activity was observed in mitochondria isolated from Sod2(-/+) mice compared with Sod2(+/+) mice, with no change in the activities of either glutathione peroxidase or copper/zinc superoxide dismutase. However, the level of total glutathione was 30% less in liver mitochondria of the Sod2(-/+) mice. The reduction in MnSOD activity in Sod2(-/+) mice was correlated to an increase in oxidative damage to mitochondria: decreased activities of the Fe-S proteins (aconitase and NADH oxidoreductase), increased carbonyl groups in proteins, and increased levels of 8-hydroxydeoxyguanosine in mitochondrial DNA. In contrast, there were no significant changes in oxidative damage in the cytosolic proteins or nuclear DNA. The increase in oxidative damage in mitochondria was correlated to altered mitochondrial function. A significant decrease in the respiratory control ratio was observed in mitochondria isolated from Sod2(-/+) mice compared with Sod2(+/+) mice for substrates metabolized by complexes I, II, and III. In addition, mitochondria isolated from Sod2(-/+) mice showed an increased rate of induction of the permeability transition. Therefore, this study provides direct evidence correlating reduced MnSOD activity in vivo to increased oxidative damage in mitochondria and alterations in mitochondrial function.