1. The kinetics of plasma noradrenaline have been determined in normal and essential hypertensive patients by intravenous injection of tritiated noradrenaline and serial mixed venous sampling.

2. The metabolic clearance rate of plasma noradrenaline in normal subjects was approximately 1·1 min−1 m−2, whereas in essential hypertensive patients it was significantly reduced to approximately 0·61 min−1 m−2.

3. Metabolic clearance rate was negatively correlated to mean arterial blood pressure and total peripheral resistances.

4. Particularly low values of metabolic clearance rate were found in two patients with congestive heart failure and one with phaeochromocytoma.

5. We propose that the access of plasma noradrenaline to the main removal mechanisms takes place in competition with the flow of unlabelled endogenous noradrenaline directly released by nerve endings. The slower removal of plasma noradrenaline in essential hypertension could then express a larger release of endogenous noradrenaline in this condition.

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