The results of this study provide a timetable of the development of the supra-spinal descending projections, which should serve as a reference for studies that describe other parameters of development in the spinal cord and brain-stem, such as enzyme-histochemistry, immunocytochemistry and neurophysiology. The present study provides essential data on the physical development of the descending projections to the spinal cord for those investigators who study the development of functional parameters, such as neurochemical and electrophysiological, in these projections.