The objective of this study was to examine the possible involvement of oxygen free radicals in the aging process. Rates of mitochondrial O2.- and H2O2 production and oxygen consumption in the kidney and the heart were compared among seven different mammalian species namely, mouse, hamster, rat, guinea pig, rabbit, pig, and cow, whose maximum life span potential (MLSP) varies from 3.5 to 30 years. The rates of mitochondrial O2.- and H2O2 generation were inversely correlated to MLSP, and directly related to specific metabolic rate and state 4 mitochondrial respiration. Results of this study indicate that under identical conditions, mitochondria from shorter-lived species produce relatively higher amounts of reactive oxygen species than those from the longer-lived species, and, thus, support the free radical hypothesis of aging.