The sciatic nerve of newborn rats was injured unilaterally. Small solid grafts of ED-12 embryonic spinal cord prelabeled with 5-bromo-2'-deoxyuridine (BrDUr) were inserted into the host's hemicord on the side of the sciatic nerve injury on PD5-12. Each graft was connected to a neuromuscular implant, which in group 1 consisted of the soleus muscle and its nerve taken from the healthy leg of the same rat pup, whereas in group 2 the soleus neuromuscular implant was taken from an immunocompatible adult rat. Six to 12 weeks later the neurons extending axons into the nerve-muscle implants were retrogradely labeled with fast blue and diamidino yellow. The embryonic origin of these neurons was ascertained by visualizing BrDUr using an immunocytochemical method. The grafts survived well and contained many BrDUr-positive neurons. No retrogradely labeled neurons were found in group 1 animals. In group 2 (animals with adult implants) numerous retrogradely labeled neurons were present in the host's neuropil. Despite the access to the target there were no BrDUr-positive cells of embryonic origin that contained the retrograde labels. Indeed no motoneuron-like cells of embryonic origin were seen in the host neuropil in any of the animals which received a graft at the age of 5-12 days. This contrasts with previous studies where the grafts were introduced into the adult spinal cord and motoneuron-like cells of embryonic origin were present in the host neuropil. Thus, it appears that the environment of the developing spinal cord is unfavorable for survival of embryonic motoneurons.