Old rats (28 months), when compared with young adults (9 months), did not show differences in activities of superoxide dismutase (SOD) or selenium-dependent and -independent glutathione peroxidases (GPx), or in levels of GSH, GSSG, GSSG/GSH and endogenous peroxidation in liver and brain. Rates of stimulated peroxidation in vitro were decreased in the livers of old rats. Old animals showed decreased levels of hepatic catalase and glutathione reductase. Nevertheless, when enzyme activities were referred to cytochrome oxidase activity these decreases disappeared, and GPx and SOD (brain) were even increased in old rats.