The vascular supply of the rat hypothalamo-hypophyseal complex was studied by aid of a double injection technique (two dyes different in colour injected successively). The vascular bed of the median eminence, the arcuate nucleus, and the retrochiasmatic area are intimately interconnected among themselves, but isolated from the vascular system of all other parts of the hypothalamus. The arterial supply of this ensemble of the medial basal hypothalamus is common from the hypophyseal arteries, via the primary plexus and the specific vascular loops of the median eminence. From here a separation of the vascular bed leads the blood flow towards the pituitary portal vessels on the one hand, and over a subependymal vascular network towards the arcuate nuclei on the other. The venous drainage of the arcuate nuclei and the retrochiasmatic area is secured by a special set of tuberal and retrochiasmatic veins. --The unity of the vascular bed of the median eminence, arcuate nucleus, retrochiasmatic area complex, with a blood flow largely in the direction indicated by this sequence, is of considerable importance for the interpretation of the results of experimental interference with this region.