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.

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