There is evidence that the endothelium plays an important role in the control of human vascular tone by releasing endothelium-derived nitric oxide. The hypothesis that an impairment of this mechanism is involved in the increased peripheral vasoconstriction of patients with chronic congestive heart failure (CHF) was tested. Acetylcholine and N-monomethyl-L-arginine (L-NMMA), a specific inhibitor of nitric oxide synthesis from L-arginine, were infused in the brachial artery of healthy volunteer subjects (controls) and patients with severe CHF. The radial artery diameter was determined by a high-precision A-mode ultrasound device, using a 10 MHz probe. Forearm blood flow was calculated from vessel diameter and blood flow velocity measured simultaneously by Doppler. The blood flow response to acetylcholine was blunted in patients with CHF compared with that in control subjects. In contrast, the decrease in blood flow induced by L-NMMA was exaggerated in CHF, and the blood flow response to nitroglycerin was preserved. The changes in radial artery diameter induced by acetylcholine and L-NMMA were not significant in control subjects and CHF patients, but dilation of the radial artery by nitroglycerin was significantly reduced in CHF. The results demonstrate an impaired endothelium-dependent dilation of forearm resistance vessels in CHF, suggesting a reduced release of nitric oxide on stimulation. In contrast, the basal release of nitric oxide from endothelium of forearm resistance vessels is preserved or may even be enhanced, and may play an important compensatory role in chronic CHF by antagonizing neurohumoral vasoconstrictor forces in CHF.