The haemodynamic effects of two vasodilators (phentolamine and nitroprusside) have been studied in 11 patients who developed acute left ventricular failure during the acute phase of a myocardial infarction. The dose was adjusted to the highest level failing to cause tachycardia (phentolamine: 0.19 to 0.76 mg/mn; nitroprusside: 0.07 mg/mn), and the treatment was maintained for between 5 hours and 8 days. Under these conditions, after one hour a diminution in mean arterial pressure can be observed (from 94 to 82 mmHg, p less than 0.005), as can a decrease in pulmonary arterial diastolic pressure (from 21 to 14 mmHg, p less than 0.001); the cardiac index rises slightly (from 3.1 to 3.51/mn/m2(NS), and the systemic and pulmonary vascular resistance are decreased by 20% and 30% respectively. The index of systolic work does not increase to a significant degree (30.7 to 32.9 g-m-syst-m2), the same work being carried out with smaller filling pressures. Together with the lessened resistance to ventricular ejection, and also, it seems, to a dedrease in venous tone, this beneficial effect has been particularly marked in a case of posterior papillary syndrome with mitral incompetence. As the response varies from individual to individual, constant monitoring of arterial pressure and cardiac rate is vital if the maximum therapeutic effect is to be achieved, particularly in hypertensive patients, in whom tachycardia in response to lowering of the arterial pressure occurs more readily.