Previous experiments in rat incisors indicate that the odontoblasts form an impermeable barrier which prevents fluid movement between pulp and dentine. The permeability of the odontoblast layer has now been investigated in pig molars which are more analogous to human teeth. The heads and necks of anesthetised piglets were perfused intra-arterially with lanthanum nitrate in Ringer's solution or with Ringer's solution alone. Molar tooth germs were removed, sliced, fixed by immersion and embedded in resin. Ultrathin sections including pulp and dentine were examined by transmission electron microscopy. Fenestrated capillaries were permeable to the electron dense lanthanum which thus entered the extracellular space between the odontoblast cell bodies. The lanthanum was excluded from predentine indicating that a barrier to permeability is present. In the above specimens and in others from 2 animals which were fixed by perfusion fixation, longitudinally oriented bundles of collagen fibrils were found passing from dentine through predentine into the odontoblast layer. Longitudinal collagen was also present between odontoblast cell bodies and entering the pulp at their basal ends. This suggests that classical von Korff fibres are present during primary circumpulpal dentinogenesis. In some sections longitudinally oriented collagen was absent. The junctions showed features of classical tight junctions but open tight junctions containing longitudinal collagen were also observed, suggesting that the junctions may modulate. Despite a trace of evidence that lanthanum can leak through adjacent to longitudinally penetrating collagen we concluded that the biological permeability barrier is maintained. The presence of the barrier indicates that other than the longitudinal collagen fibrils of which the source is unknown, all molecules incorporated into dentine are deposited there by the odontoblasts. An advantage of the barrier may be that it provides a closed environment for the orderly process of matrix deposition and mineralisation of dentine.