Aging is characterized by a general decline in physiological functions that affects many tissues and increases the risk of death. Deterioration of mitochondria, the major source and target of reactive oxygen species (ROS), is implicated in aging and a variety of age-related diseases. In the present study, the activities of citric acid cycle enzymes, such as isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, succinate dehydrogenase, and malate dehydrogenase, were found to be decreased in aged rats as well as that of electron-transferring enzymes such as NADH dehydrogenase and cytochrome c oxidase. After supplementation of carnitine to aged rats, the activities of these enzymes reverted nearer to that of young control rats. These findings suggest that L-carnitine improves the activities of mitochondrial enzymes, increases the electron flow through the electron transport chain, and improves reducing equivalence, thereby improves energy status in aged rats.