We studied the electrophysiological effects of amiodarone on isolated rabbit heart muscles by conventional microelectrode techniques. It significantly suppressed not only sinus node functions (basic cycle length, sinus recovery time, and corrected sinus recovery time) but also atrioventricular node functions (AH interval, effective refractory period, and functional refractory period) by both superfusion (10 micrograms/ml amiodarone in Tyrode solution) and long-term oral administration (50 mg/day for 2 weeks, then 25 mg/day for 4-6 weeks). On the other hand, oral administration significantly lengthened the action potential duration and effective refractory period of the left atrium and the right ventricle, while superfusion did not. The maximum rate of depolarization, action potential amplitude, and diastolic resting potential were not changed by either route of administration. It is thought that the action of amiodarone on the sinus node and on the atrioventricular node is mainly due to its noncompetitive sympathetic inhibition, and that its action on the left atrium and on the right ventricle is mainly due to reduction of serum triiodothyronine, which requires long-term administration of amiodarone.