The ventral quadrant of embryonic spinal cord with its motoneurons prelabelled by 5-bromo-2'-deoxyuridine was grafted into the spinal cord of adult rats. The ventral horn of the host had been previously partially depleted of its own motoneurons by a neonatal nerve lesion. To enhance the chances of survival of the transplanted embryonic motoneurons a target muscle was provided for their axons. Two to three months after the grafts were inserted into the cord nuclei containing 5'-bromo-2'-deoxyuridine were found in the graft and in the host's spinal cord. Many of the stained nuclei were much larger than those of embryonic motoneurons, and their size distribution was similar to that of nuclei from control motoneurons. Retrograde labelling with horseradish peroxidase, injected into the target muscle provided for the embryonic motoneurons, showed that some motoneurons had reached the muscle and presumably made contact with it. Physiological and histological examination of the target muscle showed that it was innervated and that it contained at least three different types of muscle fibres. Thus embryonic motoneurons can survive and develop in the adult spinal cord. Moreover, they seem to be able to make functional connections with skeletal muscle fibres. The heterogeneity of the muscle indicates that the motoneurons that supply them are able to differentiate into various types of cells.