Cardiomyocytes are thought to be terminally differentiated. Although a small percentage of the cells may have proliferative capacity, it is not sufficient to replace injured or dead cardiomyocytes. Death of cardiomyocytes occurs, for example, when a coronary vessel is occluded by a thrombus and the surrounding cardiomyocytes cannot be supplied with necessary energy sources from other coronary vessels. Loss of functional cardiomyocytes may lead to chronic heart failure. A potential route for restoring “normal” heart function is replacement of injured or dead cardiomyocytes by new functional cardiomyocytes. Human embryonic stem (hES) cells are a potential source of cells for cardiomyocyte replacement. Either spontaneously, or upon induction, differentiation of hES into cardiomyocytes can be achieved (1-6). Cardiomyocyte differentiation from hES cells (hES2) occurs within 12 days of co-culture with a mouse endoderm-like cell line, END-2. Based on cardiomyocyte phenotype and electrophysiology, the majority of hES-derived cardiomyocytes resemble human fetal ventricular cardiomyocytes (1, 2). However, the efficiency of cardiomyocyte differentiation from standard co-culture experiments is low.