1. Plasma dopamine β-hydroxylase (DβH) amounts were measured by radioimmunoassay in twenty-eight patients, twenty of whom had essential hypertension. There was a positive correlation between resting diastolic blood pressure and plasma DβH concentration. 2. Plasma DβH amounts also correlated significantly with those of plasma noradrenaline (NA) in individual patients. 3. These findings provide further support for the conclusions drawn from studies of plasma catecholamines that the sympathetic nervous system contributes toward the maintenance of the elevated blood pressure in essential hypertension.

1. Plasma catecholamine and dopamine β-hydroxylase (DβH) amounts were measured in a series of patients with proven phaeochromocytoma and compared with those of a group with essential hypertension. 2. Circulating catecholamine amounts were greatly elevated in phaeochromocytoma but plasma DβH was not significantly raised. 3. There was a significant correlation between plasma noradrenaline and DβH in individuals with essential hypertension but not with phaeochromocytoma. 4. The relative deficiency of DβH secretion in phaeochromocytoma indicates that the mechanism of release of catecholamines from these tumours does not involve the normal exocytosis of catecholamine storage vesicles. This supports the hypothesis that the primary defect responsible for the hypertension is excessive synthesis of catecholamines that bypass the normal storage and secretion mechanisms and diffuse into the circulation.