In 15 patients with acute myocardial infarction associated with signs of left ventricular dysfunction, phentolamine was infused intravenously in a dose of 10 mg per hour. This therapy induced a substantial reduction in mean right atrial pressure from 10 to 7 mmHg (1.3 to 0.9 kPa) (P) less than 0.001), and in pulmonary capillary wedge pressure from 20 to 13 mmHg (2.7 to 1.7 kPa) (P less than 0.001). The cardiac index increased from 2.5 to 3.0 1/min per m-minus 2 (P less than 0.001) accompanied by a fall in both the systemic and pulmonary vascular resistances (P less than 0.001). On the other hand, the mean stroke work index did not change significantly after phentolamine, because of tar resistance. With the dose used the mean arterial pressure decreased from 112 to 99 mmHg (14.9 to 13.2 kPa) (P less than 0.001). No adverse effects attributable to the drug treatment were noted. Benefits of this treatment are probably related to reduction in the impedance of left ventricular ejection and possibly to its relaxant effect on the venous tone. The drug may also improve subendocardial perfusion by decreasing left diastolic ventricular pressure. This could possibly limit extension of necrosis. Thus vasodilator therapy appears to be of particular interest in left ventricular failure complicating acute myocardial infarction, where inotropic agents may be contraindicated.