Myocardial regeneration with stem cell (SC) transplantation is an emerging therapeutic strategy to improve cardiac function after infarction. SC are pluripotent cells that can proliferate, self-renew, and differentiate into cardiomyocytes. A fundamental question is how SC acquire a cardiac functional phenotype. To address this point we used mouse embryonic SC (mES) induced to differentiate via embryo-like aggregates. In this model we identified cardiac functional markers by studying the cellular electrophysiological properties. Results show that differentiation of mES gives rise to cell aggregates featuring spontaneous contractile activity. Patch-clamp recordings show that their cellular excitability (action potential and ionic currents) closely resembles that of native cardiomyocytes. Thus, mES derived cardiomyocytes may be a useful model for investigating cardiac development and key factors involved in the control of cardiac differentiation.