Summary Because reactive oxygen species (ROS) are generally believed to play an important role in tissue injury in rheumatoid arthritis, we examined the levels of lipid peroxides, superoxide dismutase (SOD), and glutathione peroxidase (GSH-Px) in the synovial membrane, serum and liver of young (8 wk) and old (12 mo) mice with collagen-induced arthritis. In the synovial membrane, serum and liver, lipid peroxide levels of both young and old mice were increased beginning on the 3rd day after the onset of arthritis. SOD activity, which scavenges O 2 and inhibits lipid peroxidation, rose markedly in the synovial membrane of young mice in parallel with the increase in lipid peroxide levels, but not so markedly in old mice. Liver GSH-Px activity, which metabolizes already formed lipid peroxides, also rose in young arthritic mice to a greater degree than in old mice. This study suggests that in inflammatory synovial lesions, lipid peroxides are generated due to an increase in ROS concentration, with resultant cytotoxicity, and that younger animals or humans can prevent this unfavorable reaction more effectively than aged ones by enzyme induction. The hypothesis that lipid peroxides formed in the oxidative lesions of the primary organ are released into the serum, trapped by the liver and metabolized there is further supported by the present study.