The ultrastructure of contacts between axonal growth cones and glial cells in the developing pyramidal tract was examined by serial sectioning at the third cervical spinal cord segment in 0-, 2-, and 4-day-old rats. Junctional specializations, composed of synapse-like contacts and invaginations, were frequently observed at the contact zone between growth cones and glial elements. The synapse-like contacts consist of clear, round vesicles of 43 +/- 6 nm in the presynaptic growth cone, a pre- and a postsynaptic density, separated by a cleft of 12.1 +/- 0.9 nm. The invaginations consist of small protrusions of the growth cone into the glial element. The invaginated glial membrane is coated. Within the glial element, close to the invagination, frequently organelles were observed that closely resemble endosomes and prelysosomes. Therefore, it is suggested that the invagination represents a stage in endocytosis or possibly phagocytosis of the protruding part of the growth cone by the glial cell. The junctional specializations are formed by growth cones and, less frequently, by axon shafts. The targets of these specialized contacts are, in general, immature glial cells located within the tract area. Occasionally, however, invaginations were also observed into myelinating oligodendrocytes, suggesting that the population of immature target cells includes oligodendrocyte precursors. With regard to the functional significance of these temporary growth cone-glial contacts, several possibilities are discussed, including the suggestion that outgrowing pyramidal tract axons provide immature glial cells with chemical messages, which may influence the timing of glial cell maturation in the tract.