Cardioversion by rapid atrial stimulation has been carried out in 29 patients with supraventricular tachycarida and 12 patients with "coarse" atrial fibrillation. Atrial stimulation rates of 60-1200/min and electrical impulses of 6-25 mA were used. Intracardial conversion was successful in 86% of the cases presenting supraventricular tachycardia. Cardioversion was not achieved with atrial fibrillation but atrial flutter and atrial tachycardia were both found to be easily converted. Transformation of an arrhythmia into sinus rhythm or into stable atrial fibrillation with a slowed ventricular rate was the criterion for a successful conversion. Transient arrhythmias prior to an eventual stable rhythm was observed in 28% of the cases. Two mechanisms to explain the induced change in supraventricular tachycardias are discussed: (1) interruption of atrial or junctional foci (overdrive suppression) and (2) interruption of a re-entry circle by single premature beats. Cardioversion using atrial stimulation is indicated in atrial flutter, atrial tachycardias and junctional tachycardias. The method is of advantage in that it does not require anesthesia or interruption of digitalis therapy and its use involves no complications. Cardioversion using DC-shock is to be preferred in all cases of atrial fibrillation.