Programmed electrical stimulation and epicardial mapping were performed to study electrophysiologic mechanisms for induction of nonsustained polymorphic ventricular tachycardia in 29 dogs with 7-day-old myocardial infarction. Nine dogs had 15 different episodes of reproducible monomorphic ventricular tachycardia (11 sustained and 4 nonsustained) (group A). Seven episodes of polymorphic ventricular tachycardia (group B) were induced in six dogs, three of which degenerated into ventricular fibrillation. Epicardial activation time during premature stimulation at the time of tachycardia induction was more prolonged in group A than in group B (183 +/- 43 vs 123 +/- 35 msec, p less than 0.01). Furthermore, induction of ventricular tachycardia was preceded by the development of functional block in 13 of 15 group A tachycardias, but in only three of seven group B tachycardias. During polymorphic ventricular tachycardia, epicardial activation patterns were radial from the earliest epicardial activation site, similar to those observed during left ventricular endocardial pacing. QRS polymorphism was associated with a beat-to-beat variation in the earliest epicardial activation site. In conclusion, inducibility of nonsustained polymorphic ventricular tachycardia is less dependent on the epicardial activation sequence than on monomorphic ventricular tachycardia.