Morphology of ventricular ectopic complexes was examined in 1,746 ECGs of 149 patients with permanent ventricular pacemakers. In 20 of 80 patients (25%) the morphology of ventricular ectopic complexes was similar to that of the paced complex. This resulted in a ventricular couplet where the spontaneous premature complex originated at the site of pacing, possibly due to local re-entry. Repeated re-entry (two ventricular ectopic complexes in a row) occurred only in one of these patients, and none had ventricular tachycardia. These observations 1) verify the existence of a mechanism which we have postulated in explaining the association between couplets and ventricular parasystole, and 2) confirm the rare occurrence of repeated reentry in such a setting. Other observations in this studyshow that premature stimulation during right ventricular pacing may alter morphology of the ventricular complex from a left to right bundle branch block pattern in some leads.