The hemodynamic effects of the oral vasodilator trimazosin were evaluated in patients with chronic congestive heart failure (CHF), initially over a 72-hour period (13 patients), and after 3 months of maintenance therapy (seven patients). During the initial evaluation, cardiac index and stroke work index increased an average of 24% and 37% respectively, and pulmonary capillary wedge pressure decreased by 26%. These beneficial hemodynamic effects were maintained during supine bicycle exercise. After 3 months of maintenance therapy, cardiac index (+30%) and stroke work index (+38%) remained elevated, and pulmonary capillary wedge pressure was lower (-38%). Improved hemodynamics during exercise were also seen after maintenance therapy. Withdrawal of trimazosin was associated with deterioration in hemodynamics and left ventricular function. Thus trimazosin has the potential to cause sustained improvement in left ventricular function, both at rest and during exercise, in patients with chronic CHF.