AIM: To investigate the effect of insulin on vascular smooth muscle cells (VSMCs) proliferation and to evaluate the intracellular signaling pathways involved. METHODS; VSMCs separated from Sprague - Dawley rats were used in this study. The proliferation of VSMCs induced by insulin was assayed by [~3H ] - thymidin incorporation. The protein expression and activity of p -ERK1/2 were determined by immunblot and [~(γ-32)P]ATP incorporation. RESULTS: Insulin induced cell proliferation in a concentration - dependent manner. The proliferative effect of insulin on VSMCs was inhibited partly by LY294002 (48.8% ) , an inhibitor of PI -3 kinase, and the ERK1/2 inhibitor PD98059 (43.6% ) , respectively. Moreover, phosphorylation of ERK1/2 and activity of ERK1/2 induced by insulin were also inhibited partly by LY294002.CONCLUSION: PI -3 kinase and ERK1/2 are involved in insulin induced VSMCs proliferation.