In avulsion nerve injuries at the site of the entrance into the muscles, not only loss of muscle function but also nerve repair is impossible because there is no distal stump to approximate with the proximal end. When nerves are implanted directly into normally-innervated muscle a neuromuscular connection is established very rarely. Howerver, motor nerves implanted into denervated muscle consistently establish functionally effective neuromuscular junctions. For ultrastructual study of regeneration of new motor end-plate, the following operative procedure were carried out on separate groups of adult rats. The denervation of the triceps muscle plus simultaneous implantation of the peroneal nerve into the proximal one-third of the denervated muscle were done. At 1, 2, 3, 4, 5 and 8 weeks after operation, the implanted.nerves were dissected form the surrounding tissues from the point of implantation to their origin. The neuromuscular junction of tibial nerve of the contralateral (normal) side was used as control group. On transmission electron microscopy, there was an initial degeneration of the axons and regional disintegration of the myelin sheaths by 1 week after operation. At the same time, a number of Schwann cells enclosing tiny axon sprouts appeared. By 2 weeks postoperatively, the contact of neuromuscular junction and target muscle was identified. From 2 weeks after operation, the beginning of myelination was observed. One month after operation, the large axon-Schwann cell units gradually divided into individual nerve fibers. Eight weeks after operation, many nerve fibers normal in appearance were found. In conclusion, the axonal maturation coincides with the time of formation of the neuromuscular junction in the target muscles. Also, for good result we improved the surgical technique dividing (under operating microscope) the peroneal nerve in fasciculi and inserted them atraumaticully in small slots in the muscle.