Direct endocardial recording from a discrete region of slow conduction in the left ventricle was performed in a patient during sustained ventricular tachycardia. The tachycardia had a right bundle branch block and superior axis configuration with the earliest site of endocardial activation in the posterolateral left ventricle. At this site, the left ventricular electrogram during the tachycardia displayed two deflections with distinctly different responses to rapid pacing. During rapid pacing from the right ventricular apex, one deflection was transiently entrained, representing activation of the ventricle distal to a region of slow conduction in the reentrant circuit. However, the other deflection was not entrained and arose from activation proximal to this region, At a critically rapid pacing rate, interruption of the tachycardia was associated with conduction block in the region of slow conduction, as demonstrated by dissociation of the two deflections on the posterolateral left ventricular electrogram. At pacing rates that transiently entrained but did not terminate the tachycardia, decremental properties of conduction were demonstrated in the region of slow conduction, but not in the rest of the reentrant circuit. These responses of the posterolateral left ventricular electrogram to pacing during ventricular tachycardia strongly suggest that the recordings bracketed a discrete region of slow conduction in the left ventricle that was critical for the maintenance of ventricular tachycardia. Furthermore, these data demonstrate that this region of slow conduction in the left ventricle had decremental conduction properties and was the site of block during rapid pacing at rates sufficient to interrupt the tachycardia. © 1988.