The midthoracic spinal cord was transected in neonatal (N = 12), weanling (N = 9), and adult (N = 9) rats and sections were prepared at various postoperative survival periods (2 to 106 days p.o.) using a modified Golgi-Cox stain. The morphology of large lamina IX "motoneurons" was examined 0 to 5 mm rostral to the lesion border and compared with the morphology of similar cells from this region in developing and adult rats. Motoneurons examined in neonatal operates underwent the same temporal sequence of morphologic changes seen during normal development. Motoneurons in weanling operates underwent rapid morphologic changes which were most marked at the earliest period studied (2 days p.o.). These changes included a decrease in perikaryal diameter, dendritic loss, and the appearance of perikaryal and dendritic spines and varicose swellings. At the longest p.o. survival period analyzed (60 days p.o.), neurons had partially recovered but still retained somatic and dendritic spines and a reduced dendritic arbor. Motoneurons stained in adult operates showed no morphologic changes at 15 days p.o. but by 90 days p.o. somatic and dentritic spines were apparent, more similar to the morphology normally seen in the early postnatal period. We interpret these data to indicate that motoneurons surviving spinal transection are relatively unaffected by this lesion during the synaptogenic period and most severely affected when denervated near the end of this time. The more gradual appearance of regressive morphologic changes seen in adult motoneurons may indicate the continued partial denervation of these cells.