This study investigates aging-related changes in lipid peroxidation and functionality in liver and skeletal-muscle mitochondria in rats fed a diet rich in polyunsaturated fatty acids (PUFA), depending on supplementation or not with coenzyme Q(10) (CoQ(10)). Two groups of rats were fed for 24 months on a PUFA-rich diet, differing in supplementation or not with CoQ(10). At 6 and 24 months mitochondria were analyzed for fatty acid profile; hydroperoxides; alpha-tocopherol; CoQ(9;) CoQ(10;) cytochromes b, c+c(1), and a+a(3) contents; cytochrome c oxidase activity; and catalase activity in cytosol. Results of this study showed for the supplemented group an age-associated decrease in the peroxidizability index, an increase in catalase activity in skeletal muscle, and modulation of the aging-related changes in different mitochondrial electron-transport-chain components in skeletal muscle. These findings provide mechanisms to explain the effect of CoQ(10) in extending the life span of animals fed a PUFA-rich diet.