The present study examined whether a peripheral nerve (PN) graft can rescue developing motoneurons from degeneration and determined when immature motoneurons begin to express a regenerative capacity. Transplantation of a PN graft was unable to rescue motoneurons from degeneration if spinal root avulsion was performed in animals younger than P14. However, this procedure did enhance motoneuron survival when root avulsion was performed at P14 or later. Immature (P1 or P7) motoneurons were unable to regenerate their axons into the transplanted PN graft following root avulsion, whereas in older animals (P14-P28) motoneurons were able to regenerate axons into the PN graft. The percentage of regenerated motoneurons increased from P21 to P28 and was similar to that of adult animals. Therefore, the regenerative capacity of rat spinal motoneurons first begins at about P14, which seems to be critical.