Single cells or cell clusters composed of 3-10 cells were isolated from the S-A and A-V nodes of the rabbit heart by coronary perfusion of collagenase dissolved in Ca-free Tyrode solution (0.04%, for 1 hr). For comparison, atrial and ventricular cells were also isolated from the same heart. Shapes of the isolated nodal cell were either rod or round and nodal cells were slightly smaller than ventricular cells. Spontaneous activity was observed in both rod and round nodal cells. The action potentials had the configurations similar to those recorded from larger conventional preparations. The membrane current recorded from the small nodal cell clusters (5-10 cells) by the two-microelectrode voltage clamp technique showed a time course similar to that of previous recordings from conventional preparations, but the amplitude of the currents was 5-10 times smaller. The isolated cells showed normal sensitivities to both acetylcholine and epinephrine. Findings given in this study indicate that the isolated cells maintain the typical membrane characteristics of the nodal cells and that they are suitable for electrophysiological studies of the cardiac pacemaker cell.