Choroid plexuses from early (30-60 days gestation) and late (125 days) sheep foetuses were examined by various ultrastructural techniques in order to investigate possible explanations for the greater penetration of protein and non-electrolytes from blood into cerebrospinal fluid (c.s.f.), which occurs in the early foetus in contrast to later stages. The greater penetration occurs despite the presence of well-formed tight junctions between the epithelial cells and the development of some of the characteristic ion gradients between c.s.f. and plasma. A tubulocisternal system of endoplasmic reticulum appears to connect the basolateral and the apical cell surfaces in the early but not in the late foetuses. Several types of connection between the endoplasmic reticulum and the cell membrane were present in the early foetuses; these may account for some of the different permeability properties of the immature choroid plexus.