Trimazosin is a new quinazoline derivative vasodilator previously thought to act primarily on resistance vessels. Vasodilators characteristically improve hemodynamics in patients with left ventricular failure. Trimazosin in single oral doses of 100 to 300 mg was given to 6 patients with class III-IV congestive heart failure due to ischemic or primary cardiomyopathy and hemodynamics were monitored for 4 hr. Onset of action was apparent at 1 hr, with peak effects occurring at 1 to 3 hr and significant effects persisting at 4 hr. At peak, mean systemic arterial pressure fell from 91.2 +/- 4.8 (SEM) to 82.5 +/- 2.5 mm Hg (p less than 0.05), heart rate was unchanged, and cardiac index (by dye dilution) was up to 2.12 +/- 0.27 from 1.76 +/- 0.33 L/min/m2 (NS). Right heart pressures (by Swan-Ganz catheterization) were reduced, right atrial, 11.3 +/- 2.1 to 8.7 +/- 1.6 mm Hg (p less than 0.02), mean pulmonary arterial pressure, 40.7 +/- 4.6 to 33.8 +/- 4.8 mm Hg (p less than 0.01), and pulmonary arterial wedge, 30.7 +/- 4.5 to 23.8 +/- 5.0 mm Hg (p less than 0.01). Forearm venous capacitance, by occlusion plethysmography, increased from 0.83 +/- 0.18 ml/100 gm to 1.12 +/- 0.22 ml/100 gm after trimazosin (p less than 0.01). Trimazosin is an orally effective vasodilator that acts on both resistance and capacitance vessels in patients with congestive heart failure.