The general objective of this study was to further elucidate the relationship between oxidative stress and the aging process. H2O2 is known to be a progenator of reactive oxygen species, such as hydroxyl free radical, by various mechanisms involving, among others, a superoxide anion radical-driven Fenton cycle, or splitting of the 0-0 bond by hemoproteins. Effects of H2O2 administration on life span, activities of superoxide dismutase, catalase, concentrations of endogenous H2O2, and glutathione in the housefly are described. Adult male flies were given various concentrations of H2O2, ranging from 0 to 100 mM H2O2, in their drinking water. Life span was shortened by H2O2 intake except in 10 mM H2O2 administrated flies, which exhibited the longest life span. Flies administered 10 mM H2O2 also contained the highest concentration of reduced glutathione (GSH). Superoxide dismutase and catalase activities were not affected by H2O2 intake. Compensatory elevation in GSH may be responsible for the increase in life span observed in 10 mM H2O2 administered flies.