1. Two aspects of nerve-muscle development were studied in neonatal rats, the role of competition between motor neurones during the elimination of polyneuronal innervation, and the dependence of muscle fibre production upon the number of motor neurones innervating the muscle. 2. Rat lumbrical muscles were partially denervated at birth by cutting the lateral plantar nerve. Many muscles remained innervated by a single motor axon from the sural nerve. These motor units developed in the complete absence of competition from other motor units. In the adult muscles the number of innervated muscle fibres was approximately the same as at birth (about 120 muscle fibres). 3. In muscles that were totally denervated at birth, the normal post-natal production of muscle fibres was arrested. In partially denervated muscles, the production of new muscle fibres depended on the number of remaining motor units. The relationship between the total number of muscle fibres and the number of remaining motor units was fitted by a simple model. 4. The results suggest that in the lumbrical muscle, the decrease in motor unit size that occurs during normal development can be accounted for entirely by competition between motor nerve terminals. 5. The results also suggest that the normal post-natal increase in the total number of muscle fibres depends on a trophic interaction between the muscle and its innervation.