Detailed electrophysiologic studies were performed in nine patients with chronic refractory ventricular arrhythmias before and after 7 to 20 weeks (mean 11 weeks) of amiodarone therapy. The amiodarone dose at the time of the repeat study ranged from 400 to 800 mg/day. The drug reduced the sinus rate (p less than 0.001) and prolonged the sinoatrial conduction time (p less than 0.05) with some prolongation of the corrected sinus node recovery time. Intra-atrial conduction was slightly prolonged both in sinus rhythm and during atrial pacing. Anterograde conduction through the AV node was significantly prolonged both in sinus rhythm (p = 0.001) and during atrial pacing (p less than 0.005), and Wenckebach AV block was seen at significantly lower atrial pacing rates after the drug (p less than 0.005). The HV interval was prolonged both in sinus rhythm (p less than 0.05) and during atrial pacing (p = 0.001), and so was the QRS width during atrial pacing (p less than 0.005) and the QT interval in sinus rhythm (p less than 0.005) and during atrial pacing (p less than 0.005). Significant prolongation of the refractory periods in the atrium, AV node, and ventricular muscle were also seen following the drug. We concluded that the significant electrophysiologic effects of this drug throughout the heart during chronic oral use attest to its clinical effectiveness in patients with atrial and ventricular arrhythmias. With due care and despite its effects on the HV interval and QRS width, it can be used in patients with intraventricular conduction defects complicating severe organic heart disease.