From a population of 260 patients with malignant ventricular arrhythmia (ventricular fibrillation or ventricular tachycardia with syncope) we identified 52 (20%) who had infrequent ventricular premature beats during exercise testing and 48 hours of ambulatory monitoring. These patients underwent invasive electrophysiologic study utilizing programmed premature stimulation with up to three extrastimuli at currents of twice and three times middiastolic threshold. The end point for testing was nonsustained ventricular tachycardia (NSVT), defined as 3 to 20 propagated responses resulting from the last premature stimulus. A multiple response was obtained in 45 patients consisting of reproducible NSVT in 36 and sustained ventricular tachycardia in nine. The 36 patients with NSVT underwent 540 electrophysiologic tests with 18 antiarrhythmic agents. Suppression of the repetitive response was achieved in 31 of the 36 patients (86.1%). After an average follow-up of 21 months, one of 31 patients in whom the repetitive response had been abolished had recurrent arrhythmia. This contrasted with recurrence in two of the five patients in whom NSVT was still provoked. During the extensive testing, ventricular fibrillation was not induced. Sustained ventricular tachycardia occurred in 27 of the 540 tests (5.0%), but cardioversion was required in only 12 (2.2%). We conclude that NSVT constitutes a safe electrophysiologic end point for selecting an effective antiarrhythmic program in patients who have experienced malignant ventricular arrhythmia but in whom monitoring and exercise testing are inadequate to guide therapy.