The effects of intravenous infusion of the alpha- and beta-adrenoceptor blocking drug labetalol (100 mg over 10 min) on heart rate, blood pressure, and several endocrine and metabolic variables have been evaluated in 12 hypertensive patients (6 men and 6 women). Drug administration was associated with significant lowering of heart rate and systolic and diastolic blood pressure in comparison with a control study performed with saline infusion. Plasma glucose was significantly increased, while no changes were observed in serum-free fatty acids, insulin, C-peptide, and growth hormone levels. Serum prolactin concentration was significantly increased in the whole group, a marked rise occurred in females, while only a trend upward was observed in males. The acute lowering of blood pressure suggests a predominant activity on alpha-adrenoceptors following intravenous labetalol infusion, although the reduction in heart rate is consistent with the beta-adrenoceptor blocking effect of the drug. Although the reason for the increase in plasma glucose is not apparent, it might depend on the rise in norepinephrine levels observed after labetalol. Stimulation of prolactin release by intravenous labetalol is not easily attributable to interference with adrenergic receptors. The mechanism of this action of the drug is presently obscure, although a possible antidopaminergic activity of labetalol might be involved.