Embryonic stem cells will cluster and differentiate into embryoid bodies, which can develop spontaneous rhythmic contractions. From these embryoid bodies, cardiomyocytes can be isolated based on density by a discontinuous Percoll gradient. These cardiomyocytes differentiate into ventricular myocytes, which is demonstrated by the expression of the ventricular specific isoform of the myosin light chain 2 gene. In this study the functional expression of ion channels was compared between fetal cardiomyocytes (in vivo) and stem cell derived cardiomyocytes (in vitro). Sodium and calcium currents together with transient potassium currents could be detected in early developmental stages (