Abstract： AIM　To study the effects of vascular endothelial cell conditioned medium (ECCM), endothelin-1 (ET-1), endothelin-converting enzyme inhibitor phosphoramidon on the proliferation of vascular smooth muscle cell (SMC) and vascular smooth muscle cell conditioned medium (SMCCM) on the proliferation of vascular endothelial cell (EC) in vitro. METHODS　EC and SMC were isolated from rabbit aortas and cultured. The conditioned medium (CM) of EC and SMC were harvested. ECCM, ECCM conditioned in the presence of phosphoramidon, SMCCM and ET-1 were added individually to the cultures. Proliferation rate of the cells was measured with 3H-thymidine incorporation method. RESULTS　ECCM and ET-1 significantly increased the proliferation of SMC in a dose-dependent manner with a maximal effect of (190±11)% (100%ECCM) and (166±9)% (100 ng*L-1 ET-1) respectively. ECCM conditioned in the presence of phosphoramidon reduced the mitogenic effect by (33±2)%. SMCCM elicited a dose dependent decrease of cultured EC proliferation with a maximal effect of (78±3)% over basal level. CONCLUSION　ECCM and ET-1 have mitogenic effects on SMC. Phosphoramidon can significantly reduce the mitogenic effect of ECCM. SMCCM inhibits EC proliferation.