Pharmacological, histological, and electron microscope studies in five cases of nevus anemicus indicate that the immediate cutaneous abnormality is sustained adrenergic vasoconstriction. This constrictor tone cannot be overcome by a wide variety of physical and pharmacological vasodilator stimuli, but sympathetic block (carried out in two cases) renders the nevus indistinguishable from surrounding skin. That the increased tone is due, at least in part, to increased sensitivity of the involved vessels to catecholamines is suggested by the hypersensitivity of these vessels to intradermal injection of a dilute solution of levarterenol. The demonstration of a nevus in which the abnormality is pharmacological and not anatomical has important wider implications in terms of functional disorders of the skin.