Changes in motor unit organisation following nerve injury in adult and neonatal rats were compared. Motor units were studied in extensor digitorum longus muscles reinnervated after nerve injury in either neonatal or adult rats. The force developed by individual motor units was measured by stimulating ventral root filaments. After nerve section in adult rats the distribution of motor unit force was restored to normal but this did not occur following nerve crush in neonatal animals. Thus following nerve injury during the neonatal period the muscles were not only permanently weaker, but the distribution of motor unit sizes was also abnormal. Muscle fibres belonging to a single motor unit were identified histologically by the glycogen depletion method, and their fibre type and cross-sectional area measured. Although all the fibres of the same unit became histochemically homogeneous, they showed greater variation in size than normal units, suggesting that factors other than the influence of the axons control the size of muscle fibres.