Thoracic spinal cord transplanted to the lumbar region at the time of neural tube closure in the chick embryo survives and initially differentiates normally similar to in situ thoracic cord. Normal numbers of motoneurons are produced that innervate the host hindlimb musculature. In control thoracic cord approximately 70% of the motoneurons are lost by normal cell death between embryonic day (E) 6 and E11–E12. By contrast, the transplanted thoracic cord loses only about 30% of the motoneurons during this period. Transplantation of one hindlimb to the thoracic region also reduces the normal loss of in situ thoracic motoneurons. We conclude that some factor(s) associated with the increased target size provided by the hindlimbs promotes the survival of thoracic motoneurons. In contrast, by E16–E18 motoneuron numbers in the thoracic transplants decrease to below control levels. Dorsal root ganglion cells in the transplant were also initially increased (on E8) but later decreased to below control values. Hindlimb muscles innervated by thoracic motoneurons in the transplant also differentiated normally up to E10 to E12. Myotube size and numbers, muscle size and myotube types (fast versus slow) all developed normally in several thoracically-innervated hindlimb muscles. However, beginning on E14 myotube numbers and muscle size were markedly decreased resulting in muscle atrophy. Injections of horse-radish peroxidase (HRP) into the thoracic transplants labelled neurons in the host spinal cord and brainstem rostral to the transplant thereby indicating an anatomical continuity between host and transplant neural tube. Injections of HRP into specific thoracically innervated hindlimb muscles on E8 labelled distinct pools of motoneurons in the transplants. The location of the labelled motor pools were similar to the location of the motor pools serving these muscles in normal lumbar cord. For example, the sartorius motor pool, which is normally restricted to the first two segments of the lumbar cord, was located in the first two segments of the thoracic transplant. We conclude that thoracic neural tube transplanted to the lumbar region initially differentiates normally, innervates hindlimb muscles and that the thoracically innervated hindlimb muscles also differentiate normally. Subsequently, however, thoracic innervation of hindlimb muscles somehow becomes incompatible with normal development and the motoneurons and muscles eventually undergo regressive changes.