Objective. To investigate the dose response relationships of methotrexate (MTX) therapy in rat adjuvant arthritis (AA), an animal model of rheumatoid arthritis (RA). Methods. Female Lewis rats were fed a defined diet and were treated with 0, 0.3, 1, 2, 3, 5, and 10 mg MTX per week beginning 3 days after adjuvant injection and lasting 6 weeks. The presence or absence of arthritis, and its degree were measured by hindpaw edema scores, ankle widths, and radiographic and histopathologic scores. Results. The 2, 3, 5, and 10 mg MTX per week doses resulted in deaths before the end of the protocol and suppressed normal body weight gain. Tissue destruction, measured by radiographic and histopathologic scores, was reduced in a dose dependent manner with increasing MTX dose. Suppression of inflammation, measured by ankle widths and radiographic and histopathologic scores, reached a maximum at the 1 mg MTX dose and declined at higher doses. Conclusions. Suppression of tissue destruction and inflammation in rat AA does not occur in a concerted fashion as the dose of MTX increases. The implications of these findings to human disease remain to be determined.