The effects of anti-rheumatic drugs (dexamethasone 0.1 mg/kg and naproxen 5 mg/kg) were evaluated immunologically and histopathologically on type II collagen-induced arthritis in Lewis rats. Increased paw volume in the hind limbs was significantly suppressed in the groups treated with dexamethasone or naproxen, but noticeable retardation of body weight gain was observed in the group treated with dexamethasone. Serum anti-type II collagen IgG was significantly suppressed in the group treated with dexamethasone but not naproxen. Histopathological evaluation by our grading system, classification of the stages in arthritic lesion development, revealed suppression of the inflammatory changes in the tarsal joints of the animals treated with dexamethasone or naproxen. From our results, histopathological evaluation is considered to be more suitable for assessment of the efficacy of anti-rheumatic drugs on type II collagen-induced arthritis, an animal model for human rheumatoid arthritis.