Preliminary conclusions on the basis of these experiments are as follows. Plasma dopamine B hydroxylase (DBH), as measured by radioimmunoassay, appears to be a useful integrative index of sympathetic activity in essential hypertension. It correlates closely with diastolic blood pressure and plasma catecholamines both at rest and after ganglion blockade. The parallel elevation in plasma DBH and catecholamine levels in the patients with hypertension suggests that increased sympathetic activity contributed to their elevated blood pressure. The dissociation of catecholamine and DBH secretion in the phaeochromocytoma patients, on the other hand, supports the hypothesis that release of catecholamines from the tumor does not occur by exocytosis of storage vesicles, but from an excess pool of newly synthesized catecholamines that bypass the normal storage and secretory mechanisms and diffuse into the circulation (Winkler and Smith, 1968).