Abstract: Pacer mediated tachycardia (PMT) is detected while the pacemaker is in the host and automatically suppressed and at the same time an indication that tachycardia has occurred and been suppressed is produced so that corrective action can be taken. The pacemaker, operating in the DDD mode, is monitored during operation. On the detection of PMT, the post ventricular atrial refractory period (PVARP) is automatically set and maintained at a magnitude, of 500 or 550 ms, substantially greater than the time of retrograde conduction from the ventricle to the atrium on the occurrence of a pulse in the ventricle. At the same time, the operation of the pacemaker is transferred from the DDD mode to the DDT mode. This transition provides an indication the PMT has occurred so that the physician can correct the operation of the pacemaker. Where PMT has occurred and been suppressed by lengthening the PVARP, the PVARP is reset to start timing out on the occurrence of a ventricular event.