Twenty patients with recurrent sustained ventricular tachycardia (VT) underwent serial electrophysiological studies (EPS) 1) to determine the predictive value of the EPS in the selection of antiarrhythmic therapy, and 2) to establish the therapeutic efficacy of available antiarrhythmic agents. In each patient VT could be reproducibly initiated by programmed stimulation. After control EPS, the effects of several drugs (lidocaine, procainamide, quinidine, disopyramide and diphenylhydantoin) on the ability to initiate VT were assessed. An oral regimen was chosen on the basis of acute EPS and its effectiveness was evaluated by repeat EPS in 24--72 hours. Blood levels achieved acutely were used as guidelines to chronic therapy. In 14 patients the initiation of VT was prevented by the acute administration of one or more agents. In 13 of these patients, a chronic oral regimen based on these results prevented recurrence of VT with a three- to 27-month follow-up. In the remaining patient, oral therapy could not achieve blood levels of procainamide shown to be effective intravenously, and VT recurred. In six patients no single drug or drug combination was effective during acute EPS, and VT recurred in all while on therapy with the agent shown to make initiation of VT most difficult. Procainamide prevented VT in nine patients; quinidine in three patients; lidocaine in three patients; diphenylhydantoin in two patients; and disopyramide in one patient. The mean duration of EPS studies was 4.5 days. This study suggests that serial EPS provides rapid identification of successful antiarrhythmic therapy and can predict in which patients conventional therapy would be ineffective, thereby identifying patients requiring more aggressive modes of therapy.