1. We have used in vitro intracellular recordings and measurements of the contractile properties of single motor units to examine the changes in muscle innervation occurring during the post-natal development of a fast-twitch muscle in the hindlimb of the rat, the extensor digitorum longus (EDL). 2. Intracellular recordings of end-plate potentials evoked in response to graded stimulation of the nerve supply to the muscle indicate that during the first day after birth, each muscle fibre receives synaptic input from at least two motoneurones and that some muscle fibres receive as many as six such inputs. With subsequent development, most of this polyneuronal innervation is eliminated: the first singly innervated fibres are encountered on day 3; by day 18 fewer than 5% of the fibres remain polyneuronally innervated. These results show that there are quantitative differences in post-natal synapse elimination in EDL compared to its well-studied counterpart, the soleus. Although the great majority of fibres in both muscles become singly innervated at about 18 days, the first singly innervated fibres appear at least a week earlier in the EDL. None the less, synapses are lost from EDL at about half the rate they are lost from soleus. 3. The number of motor units, determined by counting the number of twitch increments produced by graded stimulation of ventral root filaments teased to contain only a few EDL motor axons, remains unchanged from an average of forty-one from post-natal day 1 to day 17. In addition, the number of muscle fibres counted in muscle cross-sections stained with an anti-myosin antibody increases less than 10% from birth to adulthood. Therefore, synapse elimination in EDL occurs with a largely constant population of muscle fibres as well as motoneurones. 4. Measurements of tensions generated by single motor units indicate that the average size of a motor unit declines from 6.8% of the muscle fibres at day 1 to 2.3% at 17 days. This result indicates that each motoneurone, on average, comes to innervate threefold fewer muscle fibres. Motor units derived from each of the spinal segments innervating the muscle undergo equivalent reductions in motor unit size, indicating that there is no segmental disproportion to synapse elimination in this muscle. At all ages, there is a large diversity of motor unit sizes in the muscle. Synapse elimination therefore appears to maintain rather than decrease this diversity.(ABSTRACT TRUNCATED AT 400 WORDS)