PURPOSE: Mesenchymal stem cells (MSCs) have pleuripotency to differentiate into various cells including cardiomyocytes and have been studied for regeneration cellular therapy after myocardial infarction (MI). We investigated effects of transplantation of committed and uncommitted MSCs on cardiac function and differentiation into cardiomyocytes in rat MI model. MATERIALS and METHODS: MSCs were obtained from femur of F344 rats. Substance –X was treated for commitment into cardiomyocytes. A myocardial infarction was created by ligation of the left coronary artery in 12-week-old F344 rats and either committed MSCs (n=12) or un-committed MSC (n=15) were injected in the border zone of MI. Media injection (n=14) and Sham operation (n=10) were also performed. MSC were labeled with DiI before transplantation. Echocardiography was performed before MI and at 1, 4 and 8 weeks after MI. Immunostaining was performed for connexin-43, cardiac troponin I, and transcription factor MF20 at 8 weeks after transplantation. RESULTS: Baseline echocardiography showed no differences among the groups. Echocardiography 8 weeks after MI showed better preservation of anterior wall thickness in the MSC transplanted groups compared to the media injection group. Furthermore, we observed better preservation of anterior regional wall motion and greater ejection fraction in the treated group compared to the untreated group and the media injection group (Table). The rate of expression was significantly higher in the treated group than the untreated group for Connexin-43 (58% vs. 13%, p< 0.05) and Troponin I (75% vs. 33%, p< 0.05). CONCLUSION: Local transplantation of MSCs in a rat MI model attenuated LV dysfunction, reduced ventricular remodeling. Commitment of MSCs prior to transplantation enhan.ced this therapeutic efficacy.