The majority of patients with sporadic pheochromocytomas (pheos) have been noted to have normal plasma dopamine-beta-hydroxylase (DBH) activity. We determined the activity of DBH in the plasma of 8 patients with pheos, secondary to multiple endocrine neoplasia Type 2 (MEN II) (medullary carcinoma of the thyroid [MCT], pheochromocytoma(s), and parathyroid hyperplasia). We also determined the activity of DBH in the pheos of six of the patients. Three of the eight patients (38%) had elevated plasma DBH in the preoperative period. After surgical resection of their pheos, the plasma DBH of two of the subjects, in whom it could be measured, returned to normal. Although two of the other patients had bilateral pheos containing large amounts of DBH and norepinephrine (NE), their plasma DBH was normal in the preoperative and postoperative periods. Despite elevated urinary vanillylmandelic acid (VMA) excretion in all eight patients, their homovanillic acid (HVA) excretion was normal. VMA reflects NE plus epinephrine (E) excretion, while HVA reflects dopamine (DA) excretion. We conclude that the majority of patients with pheos, associated with MEN II, have normal plasma DBH activity.