Abstract Electrophysiologic measurements were made in 16 patients before and after the intravenous administration of procainamide. The drug was administered at two different dose levels. The lower plasma procainamide level (6 to 7 μg/ml) caused small decreases in cycle length (848 ± 13 versus 799 ± 42 msec), sinus nodal recovery time (1,166 ± 81 versus 1,024 ± 90 msec at a paced rate of 120 beats/min) and sinoatrial conduction time (105 ± 11 to 90 ± 9 msec). Sinus or atrial echo zones, whether or not they caused supraventricular tachyarrhythmia, were abolished by procainamide in 11 of 13 instances. The higher plasma level of procainamide (10 to 11 μg/ml) generally caused greater changes in electrophysiologic measurements. Atrial refractory periods tended to increase but changed significantly only at higher plasma levels. The latter effect was blunted in patients with a prolonged sinoatrial conduction time. The A-H interval and atrioventricular nodal functional refractory period tended to decrease at higher plasma levels, but not significantly. The H-V interval increased slightly from 48 ± 2 to 52 ± 2 msec; this effect was blunted in patients with bundle branch block. It is concluded that procainamide could be useful in managing supraventricular tachyarrhythmia because it abolishes atrial and sinus echo zones that trigger supraventricular tachyarrhythmia and because its widespread effects make it capable of blocking reentry at several sites. The electrophysiologic effects or procainamide are less predictable in patients with conduction disease, and some effects occur only at high plasma levels.