Objective To observe the effect of recombinant human endostatin(rhEndostatin) on the expression of p21,cyclin D1 and CDK4 genes in synovial tissue in rats with adjuvant arthritis(AA) and explore molecular mechanisms of the inhibitory effect of rhEndostatin on proliferation of fibroblast-like synoviocytes(FLS) in AA rats. Methods Thirty-six male SD rats were used in the study. They were divided at random into normal, AA model and rhEndostatin (2.5mg/kg)treated groups. Twelve rats were taken from each group. Adjuvant arthritis was induced in rats in AA model and rhEndostatin(2.5mg/kg)treated group. The effect of rhEndostatin on the expression of p21,cyclin D1,CDK4 mRNA and cyclin D1 protein in synovial tissue in AA rats was examined quantitatively by realtime fluorescent quantitative PCR and Western blotting assays,respectively. Results The expression levels of p21, cyclinD1 mRNA and cyclinD1 protein were significantly decreased by rhEndostatin, compared with the levels in AA synovial tissues (P<0.01). However, CDK4 mRNA expression levels in synovial tissue treated with rhEndostatin were significantly higher than those in AA control samples (P<0.01). Conclusion Recombinant human endostatin decreases cyclinD1 expression levels in AA synovial tissues, which may be one of the molecular mechanisms of the inhibitory effect of rhEndostatin on AA FLS pro