The objective of this study was to further examine the relationship between oxygen free radicals and the aging process in animals. The rate of H2O2, a product of superoxide anion radical dismutation, released by liver mitochondria, was measured in the mouse, rat, guinea pig, rabbit, pig and cow as well as in flight muscle mitochondria of Drosophila melanogaster and the housefly, Musca domestica. The rate of H2O2 generation was inversely related to the maximum life span potential (MLSP) of the species. A 42-fold difference in the rate of H2O2 secretion was found among mammals and an up to 300-fold difference was noted between the insects and the mammals. Results of this study indicate that under identical conditions mitochondria from animals with low MLSP release relatively greater amounts of H2O2.