The outer blood-cerebrospinal fluid barrier is formed by leptomeningeal cells of the arachnoidea. The structures underlying this barrier are tight junctions. In contrast to the tight junctions of endo- and epithelial cells, which have been investigated by means of ultrastructural as well as by molecular methodology, equivalent studies on meningeal cells are lacking. In the present study, therefore, the ultrathin section and freeze-fracture morphology of cranial leptomeningeal cells of carp, frog, chicken and rat was investigated by quantitative morphometry. In addition, the developmental features of the meningeal barrier in chicken and rat were compared. The parameters determined were the complexity of the tight junctions, the density of strands and branchpoints and the degree of association of tight junction particles with the protoplasmatic and exoplasmatic leaflets of cellular membranes. The complexity of tight junctions was highest in chicken and lowest in frog meningeal cells, whereas intermediate values were reached in the carp and the rat. E- and P-face associations were similar in carp and frog, whereas in chicken, the P-face association and in rat the E-face association dominated. During development, tight junction complexity continuously increased up to adult stages in the chicken, whereas in the rat, the adult value was already reached at postnatal day 2. At early embryonic stages, particle insertion into tight junctions was incomplete but occurred equally into both membranous leaflets; if completed at E19 in the chicken, a redistribution of particles toward a higher P-face association was observed. In the rat, tight junction particles were redistributed toward a higher E-face association. These results are discussed in the context of blood-brain barrier induction, maintenance and regulation.