SUMMARY Changes in hemodynamic parameters following 50-degree head-up tilt were studied in a population of 56 men, including 35 subjects with sustained essential hypertension and 21 agematched normotensive controls. The increase in heart rate following tilt was similar in both groups and exhibited the same reduction in response with age. The increase in vascular resistance following tilt was strongly and positively correlated with both age and baseline vascular resistance. The latter finding was observed mainly in hypertensive subjects. The study provided evidence that differentiated responses of heart rate and vascular resistance may be observed following orthostasis. In both normal and hypertensive subjects, the age dependence of heart rate response possibly reflected differences in baroreceptor reflex control of parasympathetic and sympathetic activity. In hypertensive subjects, the vascular response was amplified with age and baseline vascular resistance, suggesting a role for structural changes of the vessels in the increased vascular response. (Hypertension 10: 29-34, 1987) KEY WORDS • essential hypertension • baroreceptor reflex mechanisms • orthostasis W HEN a normal person changes from the recumbent to the erect position, the central blood volume and heart volume are decreased and the cardiac output (CO) is reduced by about 20%.'" 3 Diminished CO accompanying reduced peripheral resistance should result in a sharp fall in arterial pressure unless reflex cardioacceleration and vasoconstriction occur at the same time because of the baroreceptor reflex compensatory mechanisms.3 For that reason, baroreceptor reflex mechanisms may be adequately evaluated using passive tilting on a tilt table from the horizontal to the erect position, 4 " 8 during which muscular pumping action of the legs 9 is minimal or absent.The mechanisms resulting in the changes in heart rate and systemic vascular resistance on standing are complex, involving baroreceptors of the high and the low pressure systems and even ergoreceptors in the skeletal muscles.3 ' ltM5 Furthermore, adequate studies