In 10 patients with the Kent's bundle W.P.W. syndrome, the action of amiodarone (5 mg/kg i.v.) on conduction along the normal and anomalous pathways was studied. The drug actions were: 1) constant increase of the anomalous pathway E.R.P., in one case with antegrade functional block; 2) constant reduction of antegrade conduction velocity along the anomalous pathway, in one case with antegrade functional block; 3) reduction of retrograde conduction velocity along the anomalous pathway in 6 cases out of 8, including one with retrograde functional block; 4) constant increase of the A-V node E.R.P.; 5) reduction of sub Hisian A-V conduction velocity in 3 cases out of 7. Episodes of reciprocating tachycardia could be initiated by atrial electrostimulation in 5 patients before administration and in 3 following administration of the drug. In connection with the electrophysiological properties of the two atrio-ventricular pathways and on the mechanism of tachycardia, the use of this drug in prevention of the circus movement tachycardia in patients with the W.P.W. syndrome is discussed.