Atrioventricular reciprocating tachycardia (AVRT) associated with the Wolff-Parkinson-White (WPW) syndrome, sometimes terminates spontaneously, generally sustains and eventually becomes drug resistant. Amiodarone is a potent antiarrhythmic drug that is sometimes effective in patients with AVRT which is resistant to conventional antiarrhythmic drugs. However, systematic studies concerning the effects of amiodarone on AVRT have not been reported. This study evaluated the effects of intravenous amiodarone on electrophysiologic variables, and on the sites and the modes of termination of AVRT. Fifteen WPW patients were studied. Nine had overt, and 6 had concealed WPW syndrome. Measurements of electrophysiologic variables and the induction of AVRT were performed by atrial and/or ventricular programmed stimulations. Amiodarone was then administered at a dose of 5 mg/kg over 5 min. The effective refractory periods (ERP) of the atrial, atrioventricular node, ventricular and accessory pathway were increased significantly by amiodarone. The conduction times of all the components were significantly lengthened by amiodarone, except for the His-ventricular (HV) interval in concealed WPW patients. AVRT was induced in all patients, and was terminated by amiodarone in 12 of 13 patients with sustained AVRT. After amiodarone, AVRT was induced in 9 patients. Spontaneous termination was observed 11 times in 3 of the 9 patients in whom AVRT was still induced. In these cases, the modes and sites of termination were the same as during the baseline state. The ERPs and conduction times of all components of AVRT, except the HV interval, were significantly lengthened by amiodarone. Amiodarone is efficacious for terminating AVRT wherever weak links exist. However, sites of spontaneous termination are not significantly affected by amiodarone. (Jpn Circ J 1999; 63: 189 - 195)