Activity of cardiac pacemaker cells is caused by a balanced interplay of ion channels. However, it is not known how the rhythmic beating is initiated during early stages of cardiomyogenesis, when the expression of ion channels is still incomplete. Based on the observation that early-stage embryonic stem cell-derived cardiomyocytes continuously contracted in high extracellular K⁺ solution, here we provide experimental evidence that the spontaneous activity of these cells is not generated by transmembrane ion currents, but by intracellular [Ca²⁺]_i oscillations. This early activity was clearly independent of voltage dependent L-type Ca²⁺ channels and the interplay between these and ryanodine sensitive Ca²⁺ stores. We also show that intracellular Ca²⁺ oscillations evoke small membrane depolarizations and that these can trigger L-type Ca²⁺ channel driven action potentials.