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Effects of verapamil (5 mg i.v.) on excitation conduction in 20 patients who already had a basic alteration (7 with ventricular pre-excitation syndrome) were studied by His electrogram, during spontaneous sinusal rhythm and atrial electrostimulation in increasing frequencies. In 4 cases, a test crossed with atropine was carried out. The effects of the drug on the effective refractory period of the A-V node and the anomalous tract in 3 patients with W.P.W. syndrome of Kent's bundle were studied. The effects were: no significant modifications in the sinusal frequency; increase in the corrected preautomatic pause only in cases with sick sinus node syndrome and serious basic compromise of the sinus node function; slowing down of A-V conduction in the supra-hisian area and increase of the effective refractory period of the A-V node; no modification of the intra-atrial conduction, or the under-hisian and intraventricular A-V conduction; uneven slowing down of conduction along the anomalous tract of the pre-excitation syndrome of the James bundle; modification of the effective refractory period of the anomalous tract of the Kent bundle type, variable from case to case. On the basis of the electrophysiological properties, the possibilities and limits of use of the drug in sick sinud node syndrome in the common supraventricular paroxymal tachycardias, in atrial fibrillation, atrial flutter and arrhythmias of the ventricular pre-excitation syndrome are emphasized.