The response of corticospinal neurons to axotomy at postnatal ages from 5 days to adulthood was studied in the golden hamster (Mesocricetus auratus). Corticospinal neurons were retrogradely labeled with fluorescent rhodamine latex beads injected into the cervical or lumbar spinal cord. A unilateral lesion of the medullary pyramidal tract was made 1-2 days later and the brains fixed 1-30 days after axotomy. Comparisons of labeled axotomized corticospinal neurons with labeled normal corticospinal neurons in the contralateral cortex showed that axotomy at 14 days or later caused cell shrinkage but not cell death. Axotomy prior to 14 days caused cell death of corticospinal neurons. More neurons died the earlier the lesion was made, culminating in virtual complete cell death of corticospinal neurons following axotomy at 5 days. Axotomy at a given age did not affect all corticospinal neurons uniformly. Lumbar projection neurons underwent cell death ranging from slight to complete following axotomy at 13 and 9 days, respectively. Cervical projection neurons, in contrast, survived axotomy after a lesion at 9 days but underwent complete cell death if the lesion occurred at 5 days. Since corticospinal axons innervate the cervical cord from postnatal days 4-8 and the lumbar cord from 10-14 days (Reh and Kalil, '81; J. Comp. Neurol. 200:55-67), the ability of corticospinal neurons to survive axotomy appears to be temporally well correlated with their innervation of spinal targets. These neurons die if their axons are cut prior to target innervation but are able to survive if axotomy occurs after their axons innervate spinal targets. The results show that plasticity in the corticospinal pathway documented in previous reports cannot take the form of regrowth of severed axons, since early lesions cause extensive corticospinal cell death. Aberrant corticospinal pathways resulting from early lesions must therefore arise from undamaged axons. Additional retrograde labeling experiments showed that the opposite cortex responded to contralateral pyramidotomy by sprouting into denervated areas of the spinal cord. Thus another source of plasticity after early pyramidal tract lesions is sprouting from corticospinal axons arising from the intact cortex.