The purpose of this study was to evaluate possible effects of the administration of an aqueous Agaricus brasiliensis extract on the oxidative state of the liver, brain, and plasma in adjuvant-induced arthritic rats, a model for human rheumatoid arthritis. Daily doses of 400 mg · kg-1 were administered by gavage for 23 days. This treatment produced significant diminutions in protein carbonylation and lipid damage in the liver, brain, and plasma. It also diminished the tissue reactive oxygen species and increased the antioxidant capacity of the plasma. Antioxidant defenses, which are diminished by arthritis, were improved by treatment with the A. brasiliensis extract, as revealed by preservation of the reduced glutathione and protein thiol levels and by the tendency of the activities of some antioxidant enzymes to normalize. The increased glucose-6-phosphate dehydrogenase activity was also almost normalized by the treatment. In addition, there were indications that A. brasiliensis can inhibit the initiation of structural events that can lead to hepatocyte necrosis. In conclusion, A. brasiliensis aqueous preparations can, in principle, be visualized as potential auxiliaries in the treatment of patients with rheumatoid arthritis as a result of their capacity to reduce oxidative stress. This effect was exerted in multiple organs, as can be judged from the results obtained in the liver, brain, and plasma. The continuous ingestion of A. brasiliensis as specific preparations or as a food supplement can possibly help to attenuate the systemic effects of rheumatoid arthritis and improve the quality of life of patients affected by this disease.