The haemodynamic effects of a new beta 1-adrenoceptor partial agonist, ICI 118,587, were studied in 10 patients with dilated cardiomyopathy and moderate to severe cardiac failure. Simultaneous right and left heart catheterisations were performed. Resting central haemodynamic indices were measured before and 15 min after an intravenous dose of 0.05 mg/kg and then 15 min after an additional intravenous dose of 0.15 mg/kg of ICI 118,587. Left ventricular performance was improved--as shown by significant increases in systolic left ventricular pressure, maximum rate of rise in left ventricular pressure (dP/dt max), and cardiac output and a decrease in left ventricular end diastolic pressure--without an undesirable increase in heart rate. The top of the dose response curve was reached after the lower dose. An important unwanted effect was seen in the most severely diseased patient, whose left ventricular systolic pressure, dP/dt max, and cardiac output decreased. This was probably due to the drug's antagonistic property supervening in this patient, who probably had high concentrations of circulating catecholamines. It is concluded that ICI 118,587 has a positive inotropic action and improves cardiac performance in patients with dilated cardiomyopathy and moderate cardiac failure, although care should be exercised when the drug is given intravenously in severe failure.