Objective: There is indirect evidence that the nitric oxide system may be impaired in hypertensive patients. The objective of this study was to examine basal nitric oxide-mediated dilation in hypertensive patients. Design: The forearm blood flow (FBF) response to noradrenaline and NG-monomethyl-L-arginine (L-NMMA), a stereospecific inhibitor of nitric oxide synthesis, was compared in seven untreated hypertensive patients and 17 normotensive controls. Methods: Drugs were infused locally into the brachial artery and FBF measured using venous occlusion plethysmography. Results: In normotensives noradrenaline (60, 120 and 240 pmol/min) and L-NMMA (1,2 and 4 μmol/min) produced similar reductions in resting FBF. In the hypertensives L-NMMA was significantly less effective than noradrenaline, such that the threshold dose for L-NMMA vasoconstriction was increased and the overall response to L-NMMA reduced. Furthermore, when noradrenaline was used as an internal control there was a significant negative relationship between the response to L-NMMA and blood pressure. When the responses to L-NMMA and noradrenaline were compared between groups, the response to L-NMMA was significantly less in hypertensives compared with normotensives, whereas there was no statistical difference in the response to noradrenaline between the two groups. Conclusions: The results suggest an abnormality of basal nitric oxide-mediated dilation in the forearm arteriolar bed of patients with untreated essential hypertension.