Electrical properties of cultured newborn rat heart cells are investigated by the use of microelectrophysiological methods. Amplitudes of resting and action potentials appear close to those of in situ heart cells. Elevated spike rate of rise reveals functional fast sodium channels. An inconstant ratio of cells exhibit pacemaker-like activity but no relationship can be established between this automaticity and the tissular origin of the cultured cells. The pulsation rate appears to be linked to the action potential duration and to the pace-maker potential slope. Spontaneous arrhythmias may occur; they are mainly caused by anomalous conduction and (or) erratic pacemaker driving. Thus heart cell cultures may be considered as a precious tool in the field of the cardiac electrophysiologal and physiopathological studies.