A new kind of cardiac pacemaker for direct heart stimulation in cases of intermittent or variable heart block is presented. The pacemaker is capable of providing stimuli either synchronously with ventricular depolarization or at a fixed rate when natural pacing fails. Thus any competition between artificial and physiological rhythm is avoided, while allowing the exploitation, from the hemodynamic viewpoint, of the surviving capability of physiologic control of the heart rate. The main features of the instrument are that the shifts from synchronous to free operation and vice versa take place in an automatic way, and no electrode besides those which provide stimulation are needed for synchronization. The mean power consumption of the pacemaker is comparable with that of conventional unsynchronized pacemakers. Thus the subcutaneous implantation is possible, with a theoretical lifetime of 30 000-hour autonomy. Schematic diagrams and waveforms are included. Copyright © 1967 by The Institute of Electrical and Electronics Engineers, Inc.