AbstractMidtgård, U.1980. Blood vessels in the hind limb of the Mallard(Anas platyrhynchos): anatomical evidence for a sphincteric action of shunt vessels in connection with the arterio‐venous heat exchange system. (Institute of Comparative Anatomy, University of Copenhagen, Denmark.) —Acta zool.(Stockh.) 61(1): 39–49.Therete tibiotarsaleis the main arterio‐venous heat exchange system in the hind limb of the Mallard. A large arterial shunt and a venous shunt allow the blood to by‐pass the rete. These shunt vessels must be able to constrict so as to direct the blood to the rete when heat conservation is needed. Using ordinary histological methods and the technique of Falck and Hillarp for demonstration of biogenic monoamines, it was shown that the arterial shunt is more muscular and receives a more dense adrenergic innervation than adjacent segments of the same vessel. Perfusion with noradrenaline before fixation revealed that the arterial shunt was able to reduce its lumen to near closure. No structure, in the ordinary sense of a sphincter, was found in the shunt vein but adrenergic nerves were scattered throughout the tunica media at the base of venous valves, suggesting that a sphincteric action at these sites i