Objective To investigate the effects of tissue inhibitor-3 of matrix metalloproteinases(TIMP-3) genetransfected vascular smooth muscle cells(VSMCs) transplantation on heart structure after acute myocardial infarction (AMI) in rats and to explore the potential mechanisms. Methods Sixty-one female Wistar rats were produced AMI models by ligating the descending left coronary artery. Fifty-four rats were survived and divided into 3 groups randomly(n=18): 0.5 ml PBS containing 1×106 TIMP-3 gene-transfected VSMCs(group A), 1×106 VSMCs(group B) or 0.5 ml PBS without cell(group C) were injected into the ischemic myocardium immediately. Ischemic myocardium samples were harvested at 1 weekafter operation. The heart structure was observed through the tissue morphologic examination. The activity of TIMP-3 gene-transfected VSMCs were measured by immunohistochemical method. Proteins of TIMP-3 and matrix metalloproteinase 9(MMP-9) were determined by Western blot. Results VSMCs were cultivated and had a high purity(98%). TIMP-3 gene was transfected into VSMCs successfully. One week after operation in groups A, B and C, the average percentage of infarction myocardium size 〖KG6〗and left ventricle free wal area were 28.73%±1.56%, 39.63%±1.84% and 46.32%±2.16% separately.Group A was significantly lower than groups B and C(P＜0.01), group B was significantly lower than group C(P＜0.01). In groups A, B and C the averageleft ventricle volume indexes were 5.27±0.21 mm3／g, 6.69±0.34 mm3／g and 9.67±0.88 mm3／g respectively. Group A was significantly smaller than groups B and C(P＜0.01), group B was significantly smaller than group C(P＜0.01). The immunohistochemical observation confirmed that the implanted VSMCs and TIMP-3 gene were survival in ischemic area. The protein content of TIMP-3 in ischemicmyocardium was significantly higher in group A (300 704.8±3 692.8) than in groups B and C(195 548.8±3 014.2，177 991.1±2 502.1)(P＜0.01), the protein content of MMP-9 in ischemic myocardium was significantly lower in group A(594 827.4±5 708.5) than in groups B and C(921 461.4±8 887.4，1 044 445.0±8 788.6)(P＜0.01). Conclusion Implanted TIMP3 gene transfected VSMCs in ischemic myocardium can conspicuously reduce the myocardium remodeling after AMI.