We tested the theory that reactive oxygen species cause aging. We augmented the natural antioxidant systems of Caenorhabditis elegans with small synthetic superoxide dismutase/catalase mimetics. Treatment of wild-type worms increased their mean life-span by a mean of 44 percent, and treatment of prematurely aging worms resulted in normalization of their life-span (a 67 percent increase). It appears that oxidative stress is a major determinant of life-span and that it can be counteracted by pharmacological intervention.