The growth of corticospinal axons into the spinal cord has been demonstrated using the autoradiographic and the Fink-Heimer silver degeneration methods. In addition, the development of several types of placing reactions have been studied. After unilateral injections of radioactive proline or unilateral ablations of the somatosensory-motor cortex, corticospinal axons were found to extend into the contralateral dorsal funiculus of the lower cervical cord at one day postnatally, into mid-thoracic segments at three days, into upper lumbar cord by five days and into coccygeal segments by nine days. Corticospinal axons are first present in the contralateral spinal gray of lower cervical cord at day 5 and in the contralateral spinal gray of lower lumbar and sacral cord by day 9. Little change in the topographical distribution or in the density of the projection is found at all levels of the spinal gray after the fourteenth postnatal day. Comparable results were found with both experimental techniques. Forelimb placing is first seen between 4 to 7 days and hind limb placing between 9 to 13 days. The last placing response to appear in both the forelimbs and the hindlimbs is placing in response to tactile and light proprioceptive stimuli. After the initial onset of the placing reactions, there is a gradual increase in the frequency and speed of the responses until 14 to 17 days postnatally, at which time the reactions appear to be mature. While a causative relationship between the growth of corticospinal axons into the spinal cord and the development of placing has not been established, a close temporal relationship has been found between first: the appearance of fore-or hindlimb placing responses and the appearance of corticospinal axons within the spinal gray at the appropriate levels of the cord and second: between the completion of the primary growth of corticospinal axons at the light microscopic level and the maturation of the placing reactions.