We assessed the electrophysiologic effects and antiarrhythmic efficacy of intravenous pirmenol in 15 patients who had spontaneous and induced sustained ventricular tachyarrhythmias. At a plasma concentration of 2.29 +/- 0.75 micrograms/ml, pirmenol decreased sinus cycle length by 11 +/- 13%, increased QRS, QTc, and HV intervals by 14 +/- 12%, 13 +/- 12%, and 22 +/- 28%, respectively, and increased atrial and ventricular effective refractory periods (ERP) by 20 +/- 14% and 7 +/- 8%, respectively. There was a greater increase in QRS duration during ventricular tachycardia and ventricular pacing than during sinus rhythm (p less than 0.005). By electropharmacologic testing, pirmenol was judged effective in six patients (40%) and was proarrhythmic in one (6%). In the nine patients in whom pirmenol was judged ineffective, the cycle length of induced VT increased by 36 +/- 15% and the associated mean arterial pressure increased by 21 +/- 14 mm Hg. The only side effects were mild hypotension and mild nausea in one patient each. Intravenous pirmenol has type IA electrophysiologic effects. It can be administered safely to patients with sustained ventricular tachyarrhythmias and is as effective as approved antiarrhythmic drugs when assessed by electropharmacologic testing.