Chest wall electrical stimuli, too weak to affect the heart, act as electrical signals to an implanted ventricular-inhibited (QRS blocking) demand pacemaker which interprets them as originating from the heart and consequently responds according to its specifications. Rapid external stimulation permits diagnostic interpretation of the spontaneous electrocardiogram by completely inactivating the implanted pacemaker. The slow random delivery of external stimuli throughout the cardiac cycle delineates the pacemaker refractory period after the emission of a pacing stimulus and after the sensing of a spontaneous beat. During apparent fixed-rate pacing the demand capability of the pacemaker may be easily seen by appropriately timed chest wall stimulation which induces the pacemaker to sense a spontaneous QRS complex. This simple technique may reveal subtle changes in pacemaker performance, and contributes to the understanding of pacemaker arrhythmias.