Many investigations have been made on age-related changes of lipid peroxidation in tissue homogenates and subcellular fractions. However, to date there are few reports on the age-related change of free radicals in living organs during aging. In our study, we investigated free radical concentration in liver and sex glands of different aged rats by using electron spin resonance (ESR) technique. A significant reduction of free radicals in liver and sex glands of old aged rats was observed when compared with those of young or middle-aged ones. The decrease of free radical generation during aging is discussed.