The endothelium of brain capillaries represents the structural basis for the blood-brain barrier in vertebrates. Individual endothelial cells are linked by a continuous belt of complex tight junctions (zonulae occludentes). Hydrophilic solutes and macromolecules are believed to cross the barrier through specific carrier mechanisms. Unspecific paracellular ionic leak is thought to be very low. In rats the blood-brain barrier is not fully developed until postnatal day 24. We investigated the ultrastructure of the developing blood-brain barrier at 5 developmental stages between embryonic day 17 and young adults. The use of high power goniometric tilting of ultrathin sections allows one to gather information about the exact relationship between two opposing membranes throughout the entire length of the cleft. Our results suggest that the maturation of blood--brain barrier interendothelial clefts is accompanied by the establishment of a characteristic ratio of 'narrow zone' (complex tight junctions) to 'wide zone' (15-20 nm), and of a typical cleft length. Membrane separation larger than 20 nm disappear and individual tight junctional contacts undergo structural changes.