1. Mean plasma noradrenaline concentration was elevated in forty-four patients with established essential hypertension. Eighteen of these hypertensive patients had resting plasma noradrenaline concentrations in the normal range.
2. Patients with endogenous depression had higher mean plasma noradrenaline concentrations but significantly lower blood pressure than patients with essential hypertension.
3. Patients with phaeochromocytoma had plasma noradrenaline concentrations twenty-eight times greater than those found in essential hypertension, but blood pressures were less than 20% higher.
4. It is concluded that excess of sympathetic drive only partly explains the level of the blood pressure in essential hypertension.