OBJECTIVE There is evidence that basal NO mediated vasodilatation is abnormal in patients with essential hypertension. Studies in animals suggest that treatment of hypertension may restore the nitric oxide system towards normal. The objective of this study was to examine basal nitric oxide mediated vasodilatation in patients with treated essential hypertension. METHODS The forearm blood flow response to noradrenaline and NG-monomethyl-L-arginine (L-NMMA), a stereospecific inhibitor of nitric oxide synthesis, was compared in 11 patients with treated essential hypertension and 18 normotensive healthy controls. The results in the treated hypertensive patients were also compared with those in a previously reported group of seven untreated patients with essential hypertension. Drugs were infused locally into the brachial artery and forearm blood flow measured using venous occlusion plethysmography. RESULTS In the healthy controls noradrenaline (60, 120, and 240 pmol.min-1) and L-NMMA (1, 2, and 4 mumol.min-1) produced similar reductions in resting forearm blood flow. In the patients with treated essential hypertension, at the same doses, noradrenaline and L-NMMA also produced similar reductions in forearm blood flow. There was no significant difference in the response to either noradrenaline or L-NMMA between the healthy controls and the treated hypertensive patients. There was also no significant difference in the response to noradrenaline or L-NMMA between the treated patients and the patients with untreated essential hypertension. However, the response to L-NMMA in all subjects correlated significantly with blood pressure, such that the responses in the treated patients lay between those of the healthy controls and the untreated patients. CONCLUSIONS Basal nitric oxide mediated vasodilatation appears to be a continuum that varies with blood pressure. Reduction in blood pressure with medication moves the L-NMMA response nearer to that seen in healthy subjects. These results suggest that treatment of hypertension may restore NO mediated vasodilatation towards normal.