In six normal male volunteers oral administration of an alpha-receptor agonist, guanfacine (1 mg/q.i.d. for 4 days), had no effect on PRL release induced by 5 mg metoclopramide iv. The same treatment with quanfacine in six other normal subjects significantly reduced PRL secretion stimulated by insulin-induced hypoglycemia (P less than 0.05). These results suggest that an adrenergic pathway, hypothalamic or extrahypothalamic, might be involved in the inhibitory control of PRL secretion.