Vascularization and expression of blood-brain barrier (bbb) associated morphological characteristics were studied by ultrastructural analysis during the embryonic development of the mouse CNS. At day 9 (E9) capillaries were only found in the perineural mesenchymal tissue exhibiting fenestrations and loosely attached pericytes. At E10 endothelial cells (EC) together with pericytes and other mesenchymal cells invaded the intraneural domain. The immigrating EC showed numerous protrusions and lost their fenestrations as soon as the new intraneural capillaries were formed. Intraneural EC showed prominent nuclei, many pinocytotic vesicles and junctional complexes which appeared tight in some places. These results indicate that from the first day of intraneural vascularization onwards, the morphological properties of the bbb are present in the early embryonic mouse cerebral cortex.