The formation of the connective tissue compartments of human sciatic and tibial nerves was studied with special reference to the maturation of the basement membranes during foetal development (11-35 weeks of gestation). All Schwann cells were surrounded by continuous basement membranes as early as at week 11, while the perineurial cells became covered by basement membranes gradually between weeks 17 and 35, as estimated by electron microscopy. The first laminin subunits detectable within the nerve were the B1, B2 and M chains. These laminin subunits were present in Schwann cell basement membrane zone at week 11, and in the perineurium at week 17 and later. Laminin A and S chains were first detected at 26 weeks in the perineurium, and at a later stage (35 weeks) on Schwann cells. In mature nerves, all these five laminin chains could be demonstrated in both Schwann cell and perineurial cell basement membrane zones, although A, S and B2 chains predominated in the perineurium, and M, B1 and B2 were the predominant chains in Schwann cell basement membranes. Beta 1 and beta 4 integrins were expressed by all Schwann cells in samples from the youngest foetuses (11-17 weeks). At 22-35 weeks, however, only a subpopulation of Schwann cells stained positively for beta 1 and beta 4 integrins. Perineurial cells expressed beta 1 integrins at all ages studied. Staining for beta 4 integrin in perineurium became detectable and intensified concomitant with the formation of structural basement membranes. The results demonstrate that Schwann cells and perineurial cells change their laminin and integrin expression profiles during the maturation of peripheral nerve.