The specific activities of superoxide dismutase (SOD) and cytochrome oxidase (COX) in light and heavy synaptic mitochondria from rat cerebral cortex at different ages (4, 8, 12, 16, 20, and 24 months) have been measured. The specific activity of COX was significantly higher in light synaptic mitochondria than in heavy ones at all ages examined. However, no significant difference during aging was shown. A marked decrease of SOD in light and heavy synaptic mitochondria during aging was observed.