Chronic heart failure (CHF) is associated with endothelial dysfunction, including impaired flow-dependent (endothelium-mediated) dilation (FDD). We have previously shown that ACE inhibition improves endothelium-mediated vasodilation in healthy volunteers. The present study was designed to determine whether ACE inhibition improves the impaired FDD in patients with CHF. Because their affinity to tissue ACE may influence the ability of ACE inhibitors to affect endothelial function, we compared the effects of quinaprilat (high affinity to tissue ACE) and enalaprilat (low affinity to tissue ACE) on FDD in patients with CHF. High-resolution ultrasound and Doppler were used to measure radial artery diameter and blood flow in patients with CHF. The effects of intra-arterial infusion of quinaprilat 1.6 microg/min (n=15) and enalaprilat 5 microg/min (n=15) were determined at rest and during reactive hyperemia (causing endothelium-mediated dilation) before and after N-monomethyl-L-arginine (L-NMMA) to inhibit endothelial synthesis of nitric oxide. Quinaprilat improved FDD by >40% (10.2+/-0.6% versus 6.9+/-0.6%; P<0.01), whereas enalaprilat had no effect. In particular, the part of FDD mediated by nitric oxide (ie, inhibited by L-NMMA) was increased by >100% with quinaprilat (5.6+/-0.5% versus 2.5+/-0.5%; P<0.01). Enalaprilat had no effect on FDD even when it was infused twice in the same dose (5 microg/min) and up to 30 microg/min. The effect of sodium nitroprusside on radial artery diameter and blood flow was similar in patients treated with quinaprilat, enalaprilat, and placebo. Quinaprilat improves FDD in patients with CHF as the result of increased availability of nitric oxide, whereas enalaprilat does not. This observation suggests that intrinsic differences exist between quinaprilat and enalaprilat that determine the ability to improve endothelium-mediated vasodilation, ie, their different affinity to tissue ACE.