In Wolff-Parkinson-White (WPW) syndrome, the two most commonly occurring arrhythmias are circus movement tachycardia (CMT) and atrial fibrillation (AF). In 70% of patients with clinically documented CMT in whom the arrhythmia could be initiated by programmed electrical stimulation of the heart, the same CMT could still be initiated after long-term oral amiodarone administration. Spontaneous clinical recurrence of the arrhythmia was, however, observed in only 10% of patients. This finding suggests that the beneficial effect of amiodarone on CMT is primarily based on the prevention of the CMT-initiating premature beat. This may also apply to atrioventricular nodal reentrant tachycardia, in which amiodarone is also extremely effective in preventing relapses. The role of amiodarone in other forms of reentrant, or ectopic, supraventricular tachycardias is less well defined. During AF in WPW syndrome, the ventricular rate is related to the duration of the anterograde refractory period of the accessory pathway. Amiodarone prolongs this value, resulting in the reduction of ventricular rate during AF. Unfortunately, in the presence of a short anterograde refractory period of the accessory pathway, amiodarone results in only a small amount of lengthening of this value. In these patients the beneficial effect of amiodarone may primarily be related to the prevention of episodes of AF. We also found that the effect of oral amiodarone on the duration of the anterograde refractory period of the accessory pathway can (1) be abolished by sympathetic stimulation with isoproterenol and (2) be predicted from the effect of ajmaline or procainamide given intravenously. These observations clearly have practical clinical implications.