Objective:To investigate the effects of soy isoflavone on the proliferation of primary cultured rat aortic endothelial cells (RAECs) induced by angiotensin Ⅱ. Methods:RAECs were cultured by tissue sticking method. Cell proliferation model was established by stimulation with AngⅡ. Cell proliferation was measured by MTT assay to observe the effects of various concentrations of soy isoflavone and NG-nitro-L-arginine methyl ester(L-NAME,100 μmol/L) on RAECs proliferation induced by AngⅡ. Nitric oxide (NO) level was measured by Griess reagent. Nitric oxide synthase (NOS) and inducible nitric oxide synthase (iNOS) levels were detected by chemical colorimetric method. mRNA expression of iNOS was measured by reverse transcription polymerase chain reaction (RT-PCR). Results:Soy isoflavone at the doses ranging from 0.1 μmol/L to 1.0 μmol/L inhibited cell proliferation in a dose and time-dependent manner. The inhibitory effects were partly blocked by 100 μmol/L of L-NAME. Soy isoflavone markedly increased NO,NOS and iNOS levels,and increased iNOS mRNA expression in RAECs. Conclusion:Soy isoflavone could inhibit RAECs proliferation induced by AngⅡ,and NO might contribute to this inhibitory effects.