In pacing on demand, electric stimulation is coupled to the QRS complex and takes place only if ventricular contraction fails to occur within the preset time interval. Pacing is cancelled if the spontaneous heart rate exceeds the preset pacemaker rate. In demand pacing, the electrode delivering the electric stimulus also monitors the electrocardiogram for purposes of programming the timing device incorporated in the pacemaker. Good endocardial contact of the electrode, therefore, becomes essential to provide an adequate signal for programming of the monitoring and timing circuits. It is recommended that once the electrodes have been placed, an endocardial ECG be obtained. Its signal should at least reach 6 mv, thus providing a 3:1 margin over the threshold of the monitoring system. Twenty-seven patients with the implanted demand pacemaker were successfully treated.