1. The number of muscle fibres innervated by individual motor neurones (motor unit size) was measured in lumbrical muscles of rats aged 0--28 days, during the period of elimination of polyneuronal innervation. Motor unit sizes were determined from twitch tension measurements combined with muscle fibre counts made from histological sections of the muscles. 2. The relative tensions contributed by individual motor units declined from about 25% of the total tension at birth, to about 9% at 28 days of age. Intracellular recordings showed that part of this decrease reflected the elimination of synapses from polyneuronally innervated muscle fibres. 3. During the same period, however, new muscle fibres were produced. The total number of muscle fibres present increased from about 500 at birth to about 950 fibres in mature muscles. 4. These two processes were offsetting: some synapses were eliminated (from polyneuronally innervated fibres) while simultaneously others were formed de novo (on newly produced muscle fibres). Quantitative measurements showed that for the first 10 days after birth, there was little change in motor unit size. Thereafter production of new muscle fibres ceased, and motor unit size decreased to the adult level. 5. It is concluded that during early post-natal development, a lumbrical motor neurone maintains a nearly constant number of synapses, but extensively reorganizes its synaptic field, retracting synapses from some muscle fibres, while forming new synapses with other fibres.