Aim To investigate the survival and differentiation of mesechymal stem cells(MSC) in infarcted myocardium induced by 5-aza-2'-deoxycytidine(5-aza-CdR) and their effects on infarction-induced heart failure of rats.The chronic development was also studied. Methods MSC of Wistar rat were cultured in vitro and the second passage MSC were incubated together with 5-aza-CdR(0.3 μmol/L) for twice.Then the induced MSC labeled with bromodeoxyuridine(BrdU) were transplanted into the infarcted myocardium of rats in the model of infarction-induced heart failure.The rats receiving serum-free medium injection were used as controls.Before transplantation and 1,2,3 month(s) after transplantation,the parameters of heart function,such as ejection fraction(EF),were examined by echocardiography and the survival and differetiation of transplanted MSC were observed by double-lable immunohistochemistry and electric microscope.Chronic changes were observed for 3 months. Results One month after transplantation,the EF value of MSC transplanted group(83.3%±8.1%,n=13)was higher than that of control group(47.2%±11.8%,n=12)(p<0.01).The heart function of MSC transplanted group was also improved compared with that before transplantation(EF=64.3%±10.4%,p<0.01).The heart function of MSC transplanted group sustained during 3 months' observation.The EF value was 83.1%±14.4%(n=7) and 86.3%±3.7%(n=6) respectively 2 and 3 months after transplantation.While in serum-free medium transplanted group,the EF value was 51.6%±11.2%(n=6) and 49.1%±7.7%(n=6) respectively 2 and 3 months after transplantation.There was significant difference between two groups 2 and 3 months after transplantation(p<0.01).The Transplanted cells were survival in myocardial scar.Double-lable immunohistochemistry showed that BrdU and TroponinT of them were positive.Electric microscope showed that they had some peculiarity of cardiomyocytes 1 month after transplantation.But the nuclear was big and the cytoplasm was little.No sarcomeres and Z-bands could be found.3 months after transplantation,Z-bands could be found in the transplanted cells. Conclusion 5-aza-CdR induced MSC could improve the function in infarction-induced heart failure of rats,and the effect could sustain during 3 months' observation.They could survive in the infarct scar of rats and differentiate into cardiomyocytes gradually.