The effect of aging on the ability of Caenorhabditis elegans to adapt to oxidative stress was examined. Oxidative stress was applied with the quinone plumbagin or with hyperoxia, both of which are expected to increase intraorganismal production of O2.- and of H2O2. Young nematodes adapted by increasing their content of superoxide dismutase (SOD) and they survived, whereas older nematodes did not induce superoxide dismutase and suffered loss of viability. It thus appears that, in C. elegans, loss of adaptability to oxidative stress, monitored in terms of induction of SOD, is a hallmark of senescence.