1. The relationships between plasma noradrenaline concentration at rest and blood pressure, as well as increase in forearm blood flow in response to a brachial artery infusion of the α-adrenoreceptor-blocking agent phentolamine, were investigated in hypertensive and normotensive subjects of similar age.

2. In 44 hypertensive patients plasma noradrenaline correlated with systolic, diastolic and mean blood pressures, but no difference in the mean plasma noradrenaline concentration was found.

3. In 11 patients and 14 normotensive subjects α-adrenoreceptor blockade resulted in a similar increase in forearm blood flow. Only in the patients, however, was this increase related to plasma noradrenaline and blood pressure.

4. In patients with established essential hypertension plasma noradrenaline can be considered to be a marker of α-adrenoreceptor-mediated vasoconstriction, which, in part, determines the height of the blood pressure.

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