Cardiomyocytes (CMs) derived from human embryonic stem cells (hESC) provide a promising tool for the pharmaceutical industry. In this study the electrical properties and maturation of hESC-CM derived using two differentiation methods were compared and the suitability of hESC-CMs as a cell model for the assessment of drug-induced repolarization delay was evaluated. CMs were differentiated either in END-2 co-culture or by spontaneous differentiation. Action potentials (APs) were recorded from cells in spontaneously beating areas using the whole-cell patch-clamp technique. The hESC-CMs exhibited predominantly a ventricular-like phenotype with heterogeneous properties. Heterogeneity was indicative of the spectrum of hESC-CM maturation from embryonic-like with AP upstroke velocities <30 V/s and maximum diastolic potential (MDP) of close to -60 mV to more mature with values >150 V/s and -80 mV, respectively. The mean MDP was -70 mV and a significant difference was observed between the two differentiation methods (-66 versus -75 mV, P < 0.001). The age of the CMs did not correlate with phenotype maturation. The addition of the hERG blocker E-4031 and the sodium channel modulator veratridine significantly prolonged the AP duration. Furthermore, proarrhythmic indices were induced. In conclusion, the main observation was the heterogeneity in electrical properties of the hESC-CMs and this was observed with both differentiation methods. One-third of the hESC-CMs exhibited fairly mature electrophysiological properties, suggesting that mature CMs could be obtained from hESCs. However, improved differentiation methods are needed to produce homogeneous mature human CMs for pharmaceutical and toxicological applications.