The method described here to differentiate mouse embryonic stem (ES) cells into cardiomyocytes is adapted from Maltsev et al. and results in a high percentage of spontaneously beating cardiomyocyte-like cells. In order to determine to what extent the differentiating ES cells resemble true cardiomyocytes, the cells were electrophysiologically characterized during differentiation, using the whole-cell variant of the patch-clamp technique. Action potentials (APs) and membrane currents were recorded and analyzed off-line to determine electrophysiological changes during development.