In 14 patients with essential hypertension, the response of the heart rate and blood pressure to infusion of norepinephrine and epinephrine separately while off (control period) or on therapy with beta-adrenergic receptor blockade was examined. By titrating dosage against the response of blood pressure and pulse rate, propranolol hydrochloride was administered orally at 160 mg/day in four divided doses. There was a significant decrease in systolic blood pressure and in pulse rate during propranolol therapy, whereas diastolic blood pressure decreased but not significantly so. Normal blood pressure (140/90 mm /g or less) was attained only in five patients. The infusion of epinephrine and then norepinephrine produced a significant increase in both systolic and diastolic blood pressure during propranolol therapy, but the magnitude of the rise was significantly greater than that attained in the control period only during epinephrine infusion. We conclude that the transient hypertensive episodes which have been observed during offive visits in some hypertensive patients treated with propranolol are due mainly to release of epinephrine.