In developing rat peripheral fibers, nodal specialization appears early, prior to myelin compaction, and is first detected as a junction between the axon and the overhanging Schwann cell process characterized by a uniformly wide (approximately 18 nm) intercellular gap containing a patchy dense substance and a cytoplasmic undercoating subjacent to the axolemma. The gap width is rather consistent but the axolemmal undercoating is more variable and lower in density than that found at more mature nodes of Ranvier, and it is also highly variable in length, ranging from 0.5 to 3 micron. The outermost Schwann cell layer is usually prominent with a large volume of cytoplasm and many organelles. In freeze-fracture replicas, modal specializations are characterized by accumulations of large (approximately 10 nm) particles in the axolemma, especially the E face, but immature nodes generally have a lower particle concentration than mature nodes. No node-like particle aggregates have been found in axons not intimately associated with Schwann cells. Mature paranodal axon-Schwann cell junctions are usually formed first by the loops closest to the node and are characterized by a 2-3 nm gap between the apposed membranes, periodic intercellular densities (transverse bands) in the gap and cisternae flattened against the junctional Schwann cell membrane. The loops further removed from the node display a wider gap containing irregularly spaced or diffuse intercellular densities, or none. Mature junctions appear relatively late in the rat, and it is not unusual to find developing nodes with several Schwann cell loops present that do not indent the axolemma significantly and are not associated with the paracrystalline pattern characteristic of the mature junctional axolemma. In such instances, the nodal particle aggregates do not have sharply circumscribed boundaries. The majority of the developing nodes are asymmetric with one paranodal segment more mature than the other.