Onset and development of descending pathways to the rat embryonic spinal cord was examined by the use of retrograde transport of horseradish peroxidase (HRP). HRP was injected in the lower thoracic segments of the spinal cord of embryos ranging in age from embryonic day (E)14.5 to E20.5. A small number of labelled cells were found in the brain stem nuclei on E14.5: they were located in medullary as well as pontine reticular formation, lateral vestibular nucleus and interstitial nucleus of the medial longitudinal fasciculus. By E15.5 labelled cells were observed in the reticular formation of the caudal part of the medulla oblongata, medullary raphe nuclei, locus coeruleus, subcoeruleus nucleus, Barrington's nucleus and central gray of the midbrain. Cells in the red nucleus and in the nucleus of the solitary tract were labelled by E 16.5 and E17.5, respectively. Thereafter, labelled cells were first found in a few other nuclei: the gracile nucleus on E19.5 and the paraventricular nucleus on E20.5. The present study demonstrated that all the major supraspinal inputs except corticospinal fibers project to the lower thoracic spinal cord by E20.5.