The gross blood supply and internal vascular organization of the sciatic nerve in the rat (and other mammals including man) have been studied by injection, dissection, and microradiographic methods, in thick sections stained for alkaline phosphatase and semithin sections, and by electron microscopy. In the rat, an anastomotic vascular connection from the arteria comitans to the medial femoral circumflex artery is illustrated. In all species studied, the microvascular network of the nerve has an unusually large calibre and wide spacing, particularly when compared with that of skeletal muscle. Arterioles and venules are present within fascicles; the arterioles are thin-walled, with only rudimentary internal elastic laminae. Endoneurial capillaries are large, continuous and nonfenestrated, with prominent (often multiple) basal laminae and an unusually complete pericyte investment. Endothelial junctions appear to vary in type; not all are tight. Pinocytosis is common. The extensively interconnected vascular network of the nerve fascicle may act as an adaptable in situ reservoir for blood in the precise maintenance of the endoneurial milieu.