Arterial compliance is determined by structural factors, as collagen and elastin, and functional factors, as vasoactive neurohormones. In order to determine whether angiotensin II contributes to decreased arterial compliance in patients with heart failure, this study tested the hypothesis that administration of an angiotensin-converting enzyme inhibitor improves arterial compliance. Arterial compliance and stiffness were determined by measuring carotid artery diameter, using high-resolution duplex ultrasonography, and blood pressure in 23 patients with heart failure secondary to idiopathic dilated cardiomyopathy. Measurements were made before and after intravenous administration of enalaprilat (1 mg) or vehicle. Arterial compliance was inversely related both to baseline plasma angiotensin II (r = -0.52; p = 0.015) and angiotensin-converting enzyme concentrations (r = -0.45; p = 0.041). During isobaric conditions, enalaprilat increased carotid artery compliance from 3.0±0.4 to 5.0±0.4 10^-10 N^-1 m⁴ (p= 0.001) and decreased carotid artery stiffness index from 17.5±1.8 to 10.1±0.6 units (p= 0.001), whereas vehicle had no effect. Thus, angiotensin II is associated with reduced carotid arterial compliance in patients with congestive heart failure and ACE inhibition improves arterial elastic properties. This favorable effect on the pulsatile component of afterload may contribute to the improvement in left ventricular performance that occurs in patients with heart failure treated with ACE inhibitors.