Sixteen patients with ventricular tachycardia (VT) or nonfatal cardiac arrest were treated with propafenone (P), 900 mg/day. Electrophysiologic studies were performed before and during therapy with P. All patients had inducible sustained VT at the baseline study. During P therapy, VT was not inducible in 1 patient, was unsustained in 1 and was harder to induce in 2 patients. P increased the cycle length of VT from 307 +/- 67 to 382 +/- 107 ms. Five patients began outpatient therapy with P, including 2 in whom VT was slowed to less than 125 beats/min. Two are arrhythmia-free during follow-up of 2 and 8 months. P significantly increased intraatrial conduction time (from 44 +/- 12 to 72 +/- 22 ms), AH interval (from 115 +/- 36 to 152 +/- 45 ms), HV interval (from 55 +/- 18 to 92 +/- 42 ms), QRS duration (from 140 +/- 36 to 180 +/- 48 ms) and QT interval (from 402 +/- 30 to 459 +/- 60 ms). P increased atrial (from 247 +/- 36 to 288 +/- 38 ms) and ventricular (from 249 +/- 20 to 277 +/- 32 ms) effective refractory periods, Sinus cycle length did not change, but the corrected sinus node recovery time increased (from 162 +/- 85 to 821 +/- 1,607 ms). P aggravated arrhythmias in 4 patients. The plasma P concentration, measured either at the time of electrophysiologic studies of when therapy was discontinued, was 753 +/- 428 ng/ml. P suppressed ventricular ectopic beats in 33% and increased them in 1 patient. P has antiarrhythmic activity against VT similar to that of other antiarrhythmic drugs and has potential for serious adverse effects in some patients.