The Cordis Omni-Orthocor model 234A, an implantable antitachycardia system, was evaluated in 13 patients. Two patients had recurrent sustained supraventricular tachycardia (SVT) and 11 had ventricular tachycardia (VT). The system was used for SVT or VT termination (group 1: SVT, 2 patients; VT, 4 patients) or for demand pacing and noninvasive electrophysiologic studies for tachycardia induction and serial electrophysiologic testing alone (group 2: VT, 7 patients). The overdriver was used successfully in 4 of 6 patients in group 1 for repeated tachycardia termination (SVT and VT) during a mean follow-up period of 18 months. One patient had 1 sustained VT episode unresponsive to pacing and 1 patient had no recurrence of tachycardia. Tachycardia termination zones varied when using the system in 2 patients receiving long-term amiodarone therapy. Eighteen noninvasive electrophysiologic studies for serial drug testing were performed, 4 in group 1 and 14 in group 2. Clinical tachycardia was induced and successfully terminated by use of the overdriver in 12 studies. It is concluded that implantable antitachycardia systems can be used successfully for noninvasive tachycardia induction and termination and for reliable serial electrophysiologic studies. Such systems provide improved patient safety and acceptability and are reasonable in cost.