Application of vasodilators in evolving myocardial infarction has not yet received wide acceptance. Heretofore, clinical trials of a variety of vasodilator agents have been conducted without regard to the phase of the disease with its changing pathologic features and altering cardiac function. A classification of these phases based on the underlying myocardial disease--ischemia, necrosis, compensation, and healing--may allow a more rational interpretation of the available information. At present, vasodilators are not recommended during the ischemic phase, although some evidence suggests that preload modification by use of nitroglycerin may be beneficial in certain instances. Vasodilator drugs productive of arterial impedance reduction are probably of the greatest value late in the necrotic phase and in the compensation and healing phases. It is in these circumstances that heart failure becomes clinically manifest and maximal short-term efficacy of this intervention can be achieved. Impedance reduction, using agents such as sodium nitroprusside, will cause a prompt improvement manifested by a reduction in left ventricular filling pressure and in pulmonary congesting pressure and an increase in cardiac output. A significant improvement in short-term survival, at least, should be expected by judicious use of vasodilators.