Blood pressure reactivity is enhanced in young black subjects through mechanisms that are poorly understood. We compared alpha-adrenergic-mediated vasoconstrictor and ss-adrenergic vasodilator sensitivity and their relation to sympathetic activity in blacks and whites. Ten healthy black (age, 29.9+/-2.4 years) and 10 white (age, 28.3+/-1.9 years) men were studied. Forearm blood flow was measured with strain-gauge plethysmography after the intrabrachial artery administration of phenylephrine (1.25 to 20 microgram/min) and isoproterenol (60 and 400 ng/min) after application of lower-body negative pressure and after a cold pressor test. Forearm and systemic norepinephrine spillover were measured with a radioisotope dilution technique. alpha-Adrenergic vasoconstriction was markedly increased (ANOVA P=0.008) and ss-adrenergic vasodilation decreased (ANOVA P=0.02) in blacks. Phenylephrine (10 microgram/min) decreased forearm blood flow by 58.0+/-2.5% in blacks but only by 26.6+/-6.0% in whites (P<0.001). Vasoconstrictor response to endogenous norepinephrine, stimulated by a cold pressor test, resulted in a higher forearm vascular resistance in blacks than in whites (107.3+/-13 versus 64.8+/-13 mm Hg. mL(-)(1). 100 mL(-)(1), P=0.03). There were no significant ethnic differences in basal or stimulated forearm or systemic norepinephrine spillover. Increased vasoconstrictor and decreased vasodilator responses in blacks were not correlated. Increased sympathetically mediated vascular tone caused by enhanced vasoconstriction and attenuated vasodilation, effects that would be additive, and not increased sympathetic activity could enhance vascular reactivity and may play a role in the pathogenesis of hypertension in blacks.