The cross-sectional area of the fibers of hindlimb muscles of rats increased 10-40 times during the first 6 weeks after birth. Denervation at birth stopped the growth of the muscle fibers. The number of satellite cells decreased, and eventually all fibers vanished. Reinnervation, if any, was poor. Partial denervation did not induce collateral reinnervation. Some denervated gastrocnemius muscles were reinnervated and after 8-12 months contained hypertrophic fibers and signs of necrosis and regeneration. When soleus muscles were completely denervated and cross-reinnervated after 4 weeks by the peroneal nerve, only half as many fibers became reinnervated after neonatal denervation as compared to muscles denervated at the age of 4 weeks. The experiments suggest that immature muscle fibers are less apt to become reinnervated than mature fibers. The few reinnervated fibers may be overloaded and therefore hypertrophy and eventually necrotize. Regeneration is abortive because satellite cells are scarce. These results may be relevant for the understanding of neuromuscular disorders with early (fetal) onset.