In a randomized study the influence of anti-arrhythmic drugs on left ventricular myocardial function was tested. Propafenon, mexiletine, tocainide, disopyramide or placebo were given over seven days on top of basic treatment with digitalis, diuretics and nitrates. Propafenon, mexiletine, tocainide and disopyramide had negative inotropic effects of 16, 17, 19 to maximally 30%, respectively. Echocardiography was especially suitable for serial assessment of left ventricular failure. Propafenon, mexiletine and tocainide are the anti-arrhythmic drugs of choice in the treatment of cardiac arrhythmias. However, the at times marked decrease in contractility must be taken into account when the decision is made whether a given arrhythmia requires treatment or not.