The present study was carried out to compare the ability of motoneurons to regenerate to functionally appropriate and inappropriate muscles, following axotomy at different stages of postnatal development. Five-, 10-, 21-day-old and adult rats of both sexes were used. In one group, the right median and radial nerves were cut and reunited. In a second group, the cut nerves were cross reunited and, in a third group the nerves were merely exposed. Following survival periods of up to one year, the extent of motoneuron regeneration through the repaired nerves was determined by injecting the retrogradely transported tracers horseradish peroxidase (HRP) and Fast Blue into the flexor and extensor muscles of the right forearm. The results were expressed in terms of the difference between the number of labelled motoneurons on the experimental side of the spinal cord and the number on the control side, the latter having been labelled by injection of HRP and Fast Blue into the muscles of the left forearm. Comparisons were then made between the groups with respect to the age at which axotomy occurred, and the target of regeneration. The results showed that when axotomy was performed in 5- and 10-day-old rats, significantly fewer motoneurons were labelled, irrespective of whether or not the target was functionally appropriate, than when axotomy was performed in adulthood. The difference was most likely due to a lower survival rate of motoneurons following axotomy in neonates. No difference was found, however, between the numbers of labelled median and radial nerve motoneurons following self- versus cross-reinnervation in any age group. This suggests that, in both adult and neonatal rats, motoneurons which survive axotomy are able to regenerate equally well to functionally appropriate or inappropriate muscles.