Migration of smooth muscle cells (SMCs) into subendothelial space and their proliferation were key events in the atherogenesis, and were influenced by a variety of factors including PDGF, FGF, IGF, EGF,etc. It was known that SMCs could synthesize and secrete growth factors in a paracrine and autocrine manner. The SMCs conditioned medium (SMCs—CM) and the SMCs—sonicate (SMCs—son) were used as the source of growth factors,and their influence on the DNA synthesis of NIH 3T3 fibroblasts by ~3H—Thymidine incorporation. The resuits showed that SMCs—CM and SMCs—son both promoted the DNA synthesis of NIH 3T3 fibroblasts. These date suggest that growth factors synthesized by SMCs could be partly secreted out cells,among which some could bind with the receptors on SMC memberane,and the rest remained within the cells,stimulating the cells' prolifeartion.