Profound vasoconstriction in the peripheral circulation is the normal response to conditions in which the arterial pressure is too low for adequate tissue perfusion, such as acute hemorrhagic or cardiogenic shock. In other conditions, the most frequent of which is septic shock,¹,² hypotension occurs as a result of failure of the vascular smooth muscle to constrict. Such so-called vasodilatory shock is characterized not only by hypotension due to peripheral vasodilatation but also by a poor response to therapy with vasopressor drugs. This syndrome has long attracted interest and defied understanding, but recent work on the function of vascular smooth . . .