Haemodynamic measurements were carried out after administration of furosemide to 10 patients suffering from acute myocardial infarction and congestive heart failure. It was observed that a transient deterioration in cardiac function (decreased cardiac output, increased enddiastolic pulmonary arterial pressure and increased pulmonary and systemic resistance) occured in the pre-diuretic stage in these failing hearts. After the onset of diuresis the haemodynamic parameters showed a reversal of the previous trends (increased cardiac output, decreased enddiastolic pulmonary arterial pressure and pulmonary resistance). The consistently lower enddiastolic pulmonary arterial pressure in the diuretic phase as compared with the pre-diuretic value ensured an improvement in cardiac haemodynamics. An attempt was made to interpret the haemodynamic results in the light of the Frank-Starling's curve.