Baroreceptor function was measured in 18 patients with essential hypertension by plotting the change in pulse interval against a phenylephrine-induced transient rise in systolic blood pressure. The influence of propranolol (160 mg daily for at least 4 weeks) on this function and on heart rate after maximal exercise was studied and correlated with the plasma propranolol level. In 13 out of the 18 patients only baroreflex resetting occurred with no change in sensitivity during propranolol administration. A definite correlation was found between the degree of beta-adrenergic blockade, expressed as the reduction in maximal exercise heart rate and the change in mean arterial pressure. No relation could be shown between plasma propranolol steady state levels and these changes. The fall in blood pressure during beta-adrenergic blockade with a low dosage of propranolol apparently does not depend on changed baroreflex sensitivity, but on the intrinsic action of this drug on beta-receptors.