Human embryonic stem cell derived cardiomyocytes (hES-CM) are thought to recapitulate the embryonic development of heart cells. Given the exciting potential of hES-CM as replacement tissue in diseased hearts, we investigated the pharmacological sensitivity and ionic current of mid-stage hES-CM (20-35 days post-plating). A high-resolution microelectrode array was used to assess conduction in multicellular preparations of hES-CM in spontaneously contracting embryoid bodies (EBs). 10 µM TTX dramatically slowed conduction velocity from 5.1 to 3.2 cm/sec while 100 µM TTX caused complete cessation of spontaneous electrical activity in all EBs studied. In contrast, the Ca channel blockers nifedipine or diltiazem (1 µM) had a negligible effect on conduction. These results suggested a prominent Na channel current, and therefore we patch-clamped isolated cells to record Na current and APs. We found for isolated hES-CM a prominent Na current (244+/-42 pA/pF at 0 mV; n=19), and a hyperpolarization-activated current (HCN), but no inward rectifier K current. In cell clusters, 3 µM TTX induced longer AP interpulse intervals and 10 µM TTX caused cessation of spontaneous APs. In contrast nifedipine (Ca channel block) and 2 mM Cs (HCN complete block) induced shorter AP interpulse intervals. In single cells, APs stimulated by current pulses had dV/dtmax of 118±14 V/s in control conditions; in contrast, partial block of Na current significantly reduced stimulated dV/dtmax (38±15 V/s). RT-PCR revealed NaV1.5, CaV1.2, and HCN-2 expression but we could not detect Kir2.1. We conclude that hES-CM at mid-range development express prominent Na current. The absence of background K current creates conditions for spontaneous activity that is sensitive to TTX in the same range of partial block of NaV1.5; thus, the NaV1.5 Na channel is important for initiating spontaneous excitability in hES-derived heart cells.