Adjuvant arthritis was induced in rats by the injection of Mycobacterium tuberculosis, and its severity was scored according to the macroscopic findings of the legs, tails, and ears. The average score so obtained was lower when the rats also received indomethacin (1.5 mg/kg/day). The depression of the albumin/globulin ratio was inhibited significantly by the administration of indomethacin. The levels of acid phosphatase and beta-glucuronidase were elevated after the injection of an adjuvant, but they decreased to some extent in rats administered indomethacin. The levels of thiobarbituric acid (TBA)-reactive substances in the sera and synovia were elevated at 2 weeks after the injection of adjuvant and decreased thereafter. In rats administered 1.5 mg/kg of indomethacin, the increase in both serum and synovial levels of TBA reactants was inhibited significantly. These observations suggest that the aggravation of adjuvant arthritis may be associated with lipid peroxidation and that indomethacin may, in part, exert its anti-inflammatory effect by preventing lipid peroxide-induced damage of the synovial membrane.