Blood flow to the hindlimb muscles of chloralose-anesthetized, paralyzed cats was monitored with an electromagnetic flowmeter on the femoral artery. The functional pathways of the sympathetic constrictor and dilator innervations to the vasculature of these muscles were determined by measuring changes in vascular conductance during electrical stimulation of 1) ventral roots T12-L7 (exit of preganglionic fibers from the spinal cord and entrance into the sympathetic chain), 2) the distally intact sympathetic chain at successive levels between the L1 and L7 ganglia (presence of caudally running vasomotor fibers in the chain at each level), and 3) isolated sympathetic ganglia L2-L7 (exit of postganglionic vasomotor fibers from the chain at each level). Our results indicate that vasoconstrictor fibers emerge from ventral roots T12-L4 with maximum functional outflow at L1-L3; the fibers course downward through the sympathetic chain to exit from the chain mainly at L5-L7 or below. In contrast, the preganglionic origin of cholinergic vasodilator fibers, tested after blocking the constrictor fibers with bretylium, is limited to ventral roots L2-L5, with maximum outflow at L4. The vasodilator fibers leave the sympathetic chain to enter the spinal nerves at the same levels as the vasoconstrictor fibers.