Effects of Bunaphtine on right atrial and ventricular monophasic action potentials were investigated in 6 patients using the technique of endocavitary recording with a suction electrode. The authors found that the drug, given intravenously in the usual therapeutic dosages, increases the total duration of MAP both atrial and ventricular, together with quite a proportional ERP prolongation. At ventricular level MAP's increase in correlated to a prevailing and strong increase of phase 3. However, the variations of the MAP's amplitude, its O dv/dt phase and the cardiac specific conduction's alterations (noted only at higher dosages) have been inconstant and poor on the whole. On the basis of these results, the mechanism of the action of the drug is discussed.