AR-C 239 is a new specific alpha 1 antagonist drug with an action similar to that of prazosin. In dogs it appears to be specific for the alpha 1 adrenoreceptor. AR-C 239 was tested in 11 patients (49 +/- 11 years old) with left heart failure, who had not received any previous treatment. The drug was infused intravenously with a stepwise increase in dose, producing three stable plasma concentration plateaus: 10 +/- 2, 51 +/- 3, and 138 +/- 55 ng/ml. Hemodynamic data were collected at time T0 before drug infusion, and half-way through each infusion. Blood pressures and cardiac output were recorded using a Millar microtip manometer (left ventricle), a Swan-Ganz catheter (pulmonary artery), and a femoral catheter, connected to a Syscomoram computer system which calculated the following parameters: work, resistances, tension-time index and contractility indices. Left ventricular ejection fraction and volumes were obtained from cineangiograms performed 30 min before drug infusion and at the end of the third infusion plateau. AR-C 239 produces a dose-dependent fall in systemic blood pressure (r = 0.539, n = 33, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)