Sensory neurons of rats survive and induce the formation of taste buds in vallate papillae when grafts of rat ganglion and mouse tongue are combined in the anterior chamber of the eyes of the immunologically deficient nude mouse. The present study was undertaken to determine if cross-species neuromuscular reinnervations also occur in grafts to nude mice. The intact extensor digitorum longus muscle was taken from 20-day-old normal rats and transplanted into the site formerly occupied by the excised extensor digitorum longus muscle of nude mice. Rat muscles were placed into an innervated or denervated graft site of nude mice, into an innervated graft site of normal mice, or into an innervated graft site of nude mice immunologically reconstituted by a thymus gland graft. Autotransplantation of the extensor digitorum longus muscle to an innervated graft site was also performed in some nude mice. Many rat muscle fibers survived 60 days in nude mice, became reinnervated, and developed into histochemically defined muscle fiber types. Some rat muscle fibers survived in nude mice in a denervated graft site but these fibers were atrophic and did not form fiber types. All rat muscle fibers were rejected by normal mice or by immunologically reconstituted nude mice. Muscle fibers in autografts of nude mouse muscle also survived, became reinnervated, and formed fiber types. The present demonstration of cross-species neuromuscular reinnervation means that different animals have similar cell recognition mechanisms involved in the formation of a neuromuscular junction and in the neurotrophic induction and maintenance of muscle fiber types. Because it is quite likely that human muscle would survive in the nude mouse, this animal might be useful for elucidating the origin or reversibility of diseased human muscle.