IGARI, T., TSUCHIZAWA, M. and SHIMAMURA, T. Alteration of Tryptophan Metabolism in the Synovial Fluid of Patients with Rheumatoid Arthritis and Osteoarthritis. Tohoku J. exp. Med., 1987, 153 (2), 79-86-Tryptophan, its metabolites and related enzyme activity in synovial fluid, blood and urine in patients with rheumatoid arthritis (RA) and osteoarthritis (OA) were measured. The levels of tryptophan, kynurenine and anthranilic acid in the synovial fluid higher in RA were than in OA, whereas the xanthurenic acid level was equal in RA and OA. Indoleamine 2, 3-dioxygenase activity in the synovial membrane was higher in RA than in OA. 5-Hydroxytryptamine (5-HT) levels in the Synovial fluid and the blood and the 5-hydroxyindole acetic acid (5-HIAA) level in the Synovial fluid were essentially the same for both diseases. However, the 5-HIAA level in the serum of RA patients was higher than in those with OA, and the 5-HIAA level in the urine of RA patients was lower than in those with OA. In addition, monoamine oxidase-A and B activity in the Synovial fluid of RA patients was decreased than in those with OA. These findings suggest that metabolism of tryptophan is altered in patients with RA.