Plasma-prolactin concentration was up to four times higher in male patients with essential hypertension than in normotensive controls. Oral administration of bromocriptine, a dopaminergic agonist, suppressed plasma-prolactin and lowered arterial pressure. It is proposed that in the hypertensive patients the raised prolactin levels reflect a defect in central dopamine control which is normalised by bromocriptine. The antihypertensive effect of bromocriptine suggests that the dopaminergic system is involved in blood-pressure regulation and that reduced central dopaminergic activity may be a factor in the maintenance of essential hypertension.