Quantitative ultrastructural analysis of the corticospinal tract (CST) at the mid-thoracic spinal level in a series of early postnatal and young adult rats reveals that the tract is initially composed primarily of morphologically immature axon shafts, growth cones, and pale neuroglial processes. The total number of axons in the tract rises quickly to a peak level up to 90% greater than that present in the adult tract; it then declines, contemporaneously with the restriction of the areal extent of the set of spinally projecting cells in the cerebral cortex. During the time of axon elimination, axons remain small and morphologically immature, and small numbers of growth cones persist. Glial cells take on more mature forms within the tract several days before axon outgrowth ceases and myelination begins at the end of the second postnatal week. The fully mature CST retains a large complement of small, unmyelinated axons.