Objective To explore effects and possible mechanisms of combination therapy with methotrexate (MTX) and low dose cyclophosphamide (CTX) intermittently on the cyclin D 1 expression in the synovium of collagen induced arthritis rats.The outcome is analyzed.Methods After CIA experimental models were successfully established in female Wistar rats,they were randomly divided into CIA model group,low-dose MTX treatment group (0.9 mg·kg-1·W-1),high-dose MTX treatment group (2.7 mg·kg-1·W-1),Jowdose CTX treatment group [24 mg·kg-1·(3W)-1],and" low-dose MTX + low-dose CTX treatment group (MTX 0.9 mg·kg-1·W-1,CTX [24 mg·kg-1·(3W)-1].All of the rats were sacrificed 24 weeks after the first immunization,the specimens of ankle joints were exposed,fixed,decalcified,wrapped and cut into slices.The Cyclin D1 mRNA levels were examined by situ hybridization.Results The mRNA level of cyelin D1 in CIA synovial lining layer was significantly higher than that of normal controls,which suggested that in situ proliferation of synoviocytes might contribute to the thickening of rheumatoid synovial lining layer.The synovial expression intensity of the mRNA level of cyclin D1 in the treatment groups was decreased evidently.The mRNA level of cyclin D1 of the combination treatment group was higher than other groups,which was statistically significant (P＜0.05).Conclusion This is the first research on the effects of MTX combined with CTX intermittently based on cell cycles in CIA animal model.Combination therapy seems to be more effective than single-agent therapy.The results suggest that combining cell cycle stage specific agent MTX with cell cycle stage nonspecific agent CTX may be synergistic in the treatment of RA.Inhibition of the increased expression of the synovial cyclin D1 mRNA maybe an important mechanism of the MTX and CTX combination treatment for RA.