The effect of aging on the monoamine oxidase (MAO) activity in tissues of mice and rabbits was evaluated. There were no alterations in the MAO activity of liver, kidney, pancreas and brain (adult mice and rabbits) or testis and heart (adult mice) with aging. There were no alterations in the norepinephrine content of the brain, kidney and heart of older mice. The present study indicates that increased MAO activity is not inevitably associated with aging in all species.