β-adrenergic receptors (βARs) are important in determining vascular tone. Stimulation of β₂ARs in the vasculature produces vasodilatation, which acts to counteract the vasoconstriction produced by endogenous sympathetic agonists. Impaired vasodilatation occurs in human hypertension and in Blacks. It is now clear that receptor polymorphism may contribute to some of the variability seen in vascular response. β₂AR haplotypes determine the desensitization response to chronic agonist exposure in vivo and result in altered vasodilatory responses. (J Allergy Clin Immunol 2002;110:S318-21.)