Prolongation or reestablishment of stem cell homing through the expression of SDF-1 in the myocardium has been shown to lead to homing of endothelial progenitor cells to the infarct zone with a subsequent increase in vascular density and cardiac function. While the increase in vascular density is important, there could clearly be other mechanisms involved. In a recent study we demonstrated that the infusion of mesenchymal stem cells (MSC) and MSC that were engineered to overexpress SDF-1 led to significant decreases in cardiac myocyte apoptosis and increases in vascular density and cardiac function compared to control. In that study there was no evidence of cardiac regeneration from either endogenous stem cells or the infused mesenchymal stem cells. In this study we performed further detailed immunohistochemistry on these tissues and demonstrate that the overexpression of SDF-1 in the newly infracted myocardium led to recruitment of small cardiac myosin-expressing cells that had proliferated within 2 weeks of acute MI. These cells did not differentiate into mature cardiac myocytes, at least by 5 weeks after acute MI. However, based on optical mapping studies, these cells appear capable of depolarizing. We observed greater optical action potential amplitude in the infarct border in those animals that received SDF-1 overexpressing MSC than observed in noninfarcted animals and those that received control MSC. Further immunohistochemistry revealed that these proliferated cardiac myosin-positive cells did not express connexin 43, but did express connexin 45. In summary, our study suggests that the prolongation of SDF-1 expression at the time of acute MI leads to the recruitment of endogenous cardiac myosin stem cells that may represent cardiac stem cells. These cells are capable of depolarizing and thus may contribute to increased contractile function even in the absence of maturation into a mature cardiac myocyte.