To better understand blood flow and edema formation in the lung, anatomic evidence for venous sphincters was sought by a scanning electron microscopic investigation of vascular casts in the rat. In the first study, indentations in the casts of veins and arteries were compared. Casts of the pulmonary veins had narrow (1 to 3 µm) circumferential constrictions about every 30 µm in length (range 20 to 50 µm) and before and after accepting tributaries. Most arteries had no constriction. Infrequently, arterial narrowing occurred over long distances (> 30 µm). To determine if these indentations could be altered by a neural stimulus, a second study tried to cast the pulmonary microcirculation after the rats were given a blow to the head, but was unsuccessful. In a third study, the pulmonary vasculature of anesthetized rats was cleared and cast. As the resin began to harden and the animals' circulation ceased, half the animals were given a sharp blow on the head. The casts of the veins in the animals that received the blow had deeper bands than the control animals (p < 0.001). Pulmonary veins have annular constrictions (sphincters) that are strategically located to influence blood flow. They deepen in response to the neural discharge of a head blow and should be considered in the study of neurogenic pulmonary edema.