A study evaluated the effects of individual administration of methionine or glucosamine and compared with the combined administration of methionine and glucosamine on the adjuvant arthritis model of rheumatoid arthritis in rats. Adjuvant arthritis was induced in female Lewis rats by injecting Freund's complete adjuvant (FCA) into the right hind paws and methionine (200 mg/kg body weight/day) and/or glucosamine (400 mg/kg/day) were orally administered for 21 days. The progression of the adjuvant arthritis was clinically evaluated for characteristic signs and symptoms by employing an arthritis score. The administration of methionine combined with glucosamine suppressed the swelling of FCA-uninjected left hind paws and the arthritis score. Additionally, histopathological examination revealed that the combined administration of methionine and glucosamine markedly suppressed synovial hyperplasia and the destruction of the cartilage surface and articular meniscus of the knee joints of FCA-injected right hind paws. Furthermore, c o m b i n e d m e t h i o n i n e a n d g l u c o s a m i n e administration suppressed the increase in the levels of nitric oxide, prostaglandin E2 and hyaluronic acid in the plasma of rats with adjuvant arthritis. By contrast, individual administration of methionine or glucosamine suppressed arthritis only slightly. These o b s e r v a t i o n s s u g g e s t t h a t t h e c o m b i n e d administration of methionine and glucosamine is more effective compared with individual administrations of methionine or glucosamine in suppressing the progression of adjuvant arthritis possibly by synergistically inhibiting synovial inflammation (identified as synovial hyperplasia and the destruction of the cartilage surface and articular meniscus) and the production of inflammatory mediators 18.