Two hundred and nineteen nonsynchronized d.c. countershocks were delivered across the intact chest of 100 unanesthetized patients and abolished 92 episodes of a variety of arrhythmias. Rhythm and conduction disturbances were observed rather frequently after countershock, especially at energy levels of 300 joules or more. Some interesting observations were the complete abolishment of ventricular multifocal activity in 7 cases and marked reduction of this activity in 4; the marked but reversible ST segment elevations at the site of application of the anterior paddle electrode (leads V2 to V3) in 36 cases and the disappearance of 4 cases of complete right bundle branch block with the restoration of normal sinoatrial mechanism. © 1966, Sage Publications. All rights reserved.