Labetalol was administered to six hypertensive subjects in increasing doses for seven days. A decrease in both supine and standing arterial pressure and heart rate was observed with no change in cardiac output and few side effects. Exercise tolerance was unaltered by the drug, but the heart rate and arterial pressure response to exercise were significantly blunted. The infusion rate of isoproterenol required to produce tachycardia was increased sevenfold by 800 mg/day labetalol and tenfold by 1600 mg/day. More than twice the control dose of phenylephrine was required during labetalol therapy to produce a rise in diastolic arterial pressure. The overshoot of arterial pressure following the Valsalva maneuver was blocked and the reflex tachycardia to amyl nitrite induced hypotension was attenuated. These studies indicate that labetalol is an antihypertensive drug that exerts both alpha and beta adrenergic blocking properties. It deserves further clinical trials in the treatment of hypertension and angina pectoris.