Abstract: AIM: To study the effects of trapidil on cell cycle of cultured rat aortic vascular smooth muscle cells (VSMC) and on the expression and activities of mitogen-activated protein kinase (MAPK) and P34^cdc2 kinase in cultured cells. METHODS: The cell cycle distribution was measured by flow cytometry. The expression of MAPK and P34^cdc2 were assayed by Western-blotting. MAPK activity and P34^cdc2 activity were assayed by phosphorylation of their specific substrates myelin basic protein (MBP) and Histone H1 after immunoprecipitation. RESULTS: Pretreatment with trapidil(Tra), the S phase in the cell cycle distribution and the mitotically active stage of the cells were markedly decreased. The enhancement of MAPK expression was markedly suppressed by Tra, but the expression of P34^cdc2 was not affected by Tra. The MAPK and P34^cdc2 activities were inhibited by Tra. CONCLUSION: Tra affects cell cycle by its inhibitory effect on the kinase activities of MAPK and P34^cdc2, and its inhibitory effect on the expression of MAPK.