The superoxide dismutase (SOD) activities in cerebral cortex of rat during aging are reported. The specific activity of cytosol SOD (Cu/Zn dismutase) decreases progressively from the 1st to the 30th month of postnatal life, whereas the specific activity of mitochondrial SOD increases with age. The alterations in the two enzymatic forms of SOD in the cerebral cortex of rat during aging may be due to separate regulation mechanisms.