After injury to the peripheral nerve in rat pups at 5 days of age the development of the fast muscles tibialis anterior and extensor digitorum longus is impaired. Whether the length of time during which the muscles are denervated affects the degree of impairment was studied here. In one group of animals the peroneal nerve was crushed near to the muscles in one leg and further away from the muscles in the other leg. In another group of animals the sciatic nerve was crushed in one leg at 5 days and in some of these animals the nerve was crushed again 5-7 days later. The recovery of TA and EDL was measured by recording the weight and tension developed once reinnervation was complete. When the nerve was crushed close to the muscles, the muscles recovered significantly better than when the site of injury was further away, while delaying reinnervation by crushing the sciatic nerve a second time, impaired recovery of the muscles. It is concluded that the permanent impairment of fast muscles seen after neonatal nerve injury depends upon the length of time that the muscles are separated from their motoneurones.