In the intact cat desynchronised sleep induces arterial hypotension and mesenteric vasodilatation, whereas in limb muscles vasoconstriction occurs because of the concurrent action of a spinal vasoconstriction reflex. After sino-aortic denervation arterial hypotension is exaggerated and mesenteric vasodilatation is greater, while the muscular bed of the limbs also becomes vasodilated rather than constricted. It is concluded that during desynchronised sleep the sino-aortic afferents act to check a central influence inhibiting vasoconstrictor tone both in the viscera and muscles: thus an excessive fall in blood pressure is prevented by avoiding a too large visceral vasodilatation and by unmasking the vasoconstrictor action of the spinal reflex in the limbs.