The mechanism of action of a vasodilator drug is complex and depends on its predominant site of action: arterial or venous system. it leads to a) complex alteration of ventricular load that changes cardiac output, b) alteration of myocardial energetic metabolism. To classify a vasodilator drug, it is useful to study: a) its mechanism of action on the peripheral vascular system, b) the left ventricular function, and c) the coronary blood flow and the myocardial metabolism. The peripheral action of the drug is assessed by simultaneous measurement of peripheral blood flow, arterial pressure and venous pressure. From these data, arterial resistance (AR) and venous tone (VT) are calculated. A change of AR and for VT permits to classify a vasodilator drug as arterial, venous or both arterial and venous. Changes of factors of ventricular load are appreciated by measurement of aortic pressure, left ventricular pressure and ventricular volumes. If no change of heart rate occures, modification of stroke volume and cardiac output is due to a reduction of end diastolic volume (venous vasodilator drug) or of end systolic volume (arterial vasodilator drug).