To determine whether high current strength pacing at the site of origin of ventricular tachycardia (VT) could prevent induction of VT, we studied 11 VTs in 10 patients with chronic coronary artery disease. The left ventricular site of origin of all VT was determined by endocardial catheter mapping. Reproducible VT induction from the right ventricular apex or outflow tract was demonstrated with a pacing current strength equal to twice diastolic threshold (.2.0 mA) with single (two VTs), double (eight VTs), or triple (one VT) extrastimuli following 8 beats of a drive cycle length of 400 to 600 msec. After determination of the baseline VT induction zone (range 10 to 80 msec), repeat induction was attempted while simultaneous pacing was performed during the 8 beat drive train from the left ventricular site of origin with the use of a high current strength (10 mA [two VTs] or 20 mA [nine VTsJ) and from the baseline right ventricular site with a current strength equal to twice diastolic threshold. Extrastimuli were introduced only from the right ventricular site over the same range of coupling intervals that resulted in VT initiation during the baseline state. In five of the 1 1 trials, no VT could be initiated; in one trial, the VT induction zone was decreased from 80 to 10 msec; in three trials, only VT of a different morphology and a distinct (> 4 cm distant) site of origin was initiated; and in two trials, VT of the same morphology was initiated. In four of the five trials in which all VT was prevented by simultaneous pacing with a high current strength at the site of origin, simultaneous pacing at a lower current strength (twice diastolic threshold) at the site of origin (three VTs) or with equally increased current strength (10 to 20 mA) at nonsites of origin (two VTs) did not prevent initiation. We conclude that: (1) high current strength pacing at the site of origin during the drive train can inhibit VT induction with extrastimuli and, (2) successful prevention of VT may depend on the pacing site being the site of origin and the current strength used during pacing.