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The acute haemodynamic effects of prazosin 5 mg were investigated in 11 patients with severe chronic congestive cardiac failure. Six patients had their antifailure therapy discontinued while five continued on their usual medication. Two patients were exercised before and after prozosin. Following the drug, pulmonary capilary wedge pressure diminished 7 +/- 2(SE) mmHg (P < 0.005) and cardiac index increased 17 +/- 6% (P < 0.02). Mean arterial pressure fell in all subjects (- 9 +/- 2 mmHg; P < 0.001) and systemic vascular resistance was reduced in all but one (- 19 +/- 5%; P < 0.005). There was no significant change in heart rate. Four of the five subjects whose regular medications were continued were evaluated by clinical examination, chest X-ray, echocardiogram and treadmill stress test, before and after four weeks of prazosin therapy, one subject having died in this time. Two improved, one deteriorated and one remained unchanged. Thus, chronic prazosin therapy benefited individual patients but did not consistently provide long term improvement.