The microvascular architecture of the subependymal matrix in premature infants was studied with microangiography and benzidine stains. This revealed that the subependymal matrix is the end zone or the border zone between cerebral arteries and the collection zone of the deep cerebral veins. Focal hypoxic changes of this subependymal matrix may occur in hypoxemia and ischemia because of the characteristic architecture. The vascular permeability of these vessels was studied in rabbits using three different molecular weights of FITC-dextran. Vascular permeability was increased in the subependymal matrix by hypoxia and especially by hypoxia associated with an increased venous pressure. These findings may be related to the pathogenesis of subependymal hemorrhage in prematurity.