1. Plasma noradrenaline was measured in fiftynine patients with mild to severe essential hypertension and in fifteen normotensive control subjects under basal and orthostatic conditions.
2. In patients with essential hypertension mean plasma noradrenaline concentrations were significantly higher than in control subjects under basal and orthostatic conditions.
3. In patients with essential hypertension basal diastolic blood pressure correlated closely with the corresponding plasma noradrenaline concentrations.
4. Long-term treatment with prindolol of patients with essential hypertension led to a significant fall in diastolic and systolic blood pressure and heart rate and to a significant decrease in plasma noradrenaline concentrations under basal and orthostatic conditions.
5. The adrenergic response to upright posture, reflected by an increase in plasma noradrenaline, was not abolished by prindolol.
6. It is concluded that the anti-hypertensive effect of prindolol in patients with essential hypertension is at least partially mediated by a decrease of sympathetic nervous activity.