To the Editor:— Two different types of pacemakers have been built that avoid competition between the electrical impulse and idiocardiac contractions. One of these is the "standby" pacemaker (of which there are two models). We had the privilege of publishing our first experience with this pacemaker inThe Journal, and subsequently elaborated on this experience elsewhere.1-3At the same time others developed a "demand" pacemaker.4 The article by Goetz (205:657, 1968) as well as your editorial, "Apace With Pacemakers," continued to refer to demand pacemakers without reference to "standby" pacemakers, as if the two were somehow different and unrelated. Despite the slight difference in mode of operation of "standby" and "demand" units, the distinction between them is largely semantic. In both cases a spontaneous idioventricular contraction will affect the next scheduled pacemaker impulse to prevent the implanted pacemaker from firing a stimulus into the vulnerable period of