The possible effect of amiodarone administration on the effectiveness and complications of electrical cardioversion of supraventricular tachyarrhythmias has not been properly assessed. To investigate the effects of amiodarone on cardioversion, we performed 130 electrical cardioversion procedures in 116 patients who were receiving long-term amiodarone therapy (group I) and 44 cardioversion procedures in 43 patients who were receiving intravenous infusions of amiodarone (group II). All patients in groups I and II had atrial fibrillation or flutter. In group I, there was a higher incidence of ventricular premature beats than in a control group of patients who underwent 100 cardioversions; one patient had severe bradycardia with asystole, which was resolved satisfactorily. In group II there was a higher incidence of sinus bradycardia and ventricular premature beats. It was concluded that electrical cardioversion of supraventricular arrhythmias can be safely performed in patients who are receiving long-term oral or intravenous amiodarone therapy if the usual precautions are observed.