Holter monitoring in a 75-year-old man with a VVI pacemaker with rate hysteresis and concomitant quinidine treatment documented the occurrence of several episodes of non-sustained polymorphous ventricular tachycardia, triggered by each first paced beat following the longer escape interval. These arrhythmias disappeared when quinidine was withdrawn or when the pacemaker was reprogrammed without hysteresis. We hypothesize that the association of the different effects produced by hysteresis and quinidine created the electrophysiologic substrate for the observed arrhythmias.