In general, the invention is directed to methods for electrically stimulating transplanted biological material, such as transplanted cells, to improve the performance of the heart during systole. In particular, the invention is directed to devices and techniques for stimulating biological material transplanted in a myocardium of a heart during an ejection phase of a cardiac cycle. The transplanted biological material may include cells, such as skeletal myoblasts, precursor cells, endothelial cells, differentiated or undifferentiated stem cells, undifferentiated contractile cells, fibroblasts and genetically engineered cells. In general, stimulation of the transplanted biological material during the ejection phase of the cardiac cycle, when the aortic and pulmonary valves are open, provides hemodynamic assistance to the heart.