A local anesthetic, methyl-bupivacaine was injected into the planta of adult mice, and the ultrastruct of motor end-plates was studied during the degenerative and regenerative cycle induced in lumbrical muscles. Muscle degeneration took place during the first day after drug administration. The postsynaptic part of the neuromuscular junction completely degenerated as did the whole injured muscle fibre. Nerve terminals, however, remained unaffected. By the second day after muscle injury, axon terminals were enclosed within Schwann cell cytoplasm and thus became separated from the residual sarcolemmal tube. One to three days later, when myotubes were formed by fusion of the surviving myoblasts, the layer of Schwann cell cytoplasm on nerve terminals was discontinuous. Subsequently nerve terminals approached the regenerating muscle cell and the subneural apparatus began to differentiate. Slight depressions and furrows appeared on the myotube surface below the nerve ending and the myotube membrane, covered with basement membrane, became undercoated by dense material in this region. Where the distance between nerve ending and myotube was reduced to that found in the normal neuromuscular junction, i.e. to about 500 A, junctional folds were formed. Fourteen days after drug administration, newly formed end-plates were indistinguishable from those in normal control lumbrical muscles.