The development of the endothelial system of the rat cerebellum was studied by immunohistochemical techniques with two antibodies specific of two membrane antigens characteristic of 'mature' endothelial cells. It appears that the maturation of endothelial cells is very late in the rat cerebellum. This absence of a fully endothelial barrier in cerebella of young rats could explain the extreme sensitivity of this part of the central nervous system to various agents.