The pharmacokinetics of prazosin (Minipress) were studied in nine patients with NYHA Class 3 or 4 congestive heart failure and in five healthy controls. After a single 5 mg oral dose, plasma concentrations of prazosin, as reflected in the area under the plasma concentration-time curve (AUC) and prazosin plasma half-life, were approximately double in the patients in comparison to the control group. Reduction in hepatic blood flow, altered gastrointestinal absorption of the drug or diminished intrinsic hepatic metabolic activity in the patient group may have contributed to the observed changes in prazosin disposition. The finding of higher prazosin plasma concentrations in patients with refractory heart failure demonstrates the need for close monitoring of these individuals following administration of the drug in the treatment of chronic congestive heart failure.