The vascular pattern of the median eminence was studied in fetal and mature rhesus monkeys. Five types of vascular structure were found including short and long loops, complex capillary structures fed by an arteriole, a large centrally-placed formation made up of capillaries and venules, and a series of simpler capillary complexes lying caudally. The large dorso-lateral complexes communicate with the capillary bed of the hypothalamus. Portal vessels drain the various formations; there may be some relationship between given zones of the hypothalamus and specific parts of the pituitary. The vascular formations were well developed in the fetuses by the last quarter of pregnancy.