A study was made of the decline in the number of motor neurons and axons of the brachial spinal cord of the rat during postnatal development. The injection of horseradish peroxidase (HRP) into the biceps muscle showed that it was innervated by motor neurons located in the dorsolateral position of the lateral motor column in segments C5 and C6; HRP injections into the triceps muscle showed that it was innervated by motor neurons located in the ventrolateral position of the lateral motor column in segments C7 and C8. There was no change in the position of these motor neuron pools between birth and maturity. However, there was a decline in the number of neurons in each pool during the postnatal period; over 35% of the neurons present at birth had disappeared by maturity. This loss of neurons was uniform throughout the rostrocaudal extent of each pool. It was accompanied by a similar percentage loss in the number of axons in a ventral root at the branchial level (C8). Electrophysiological measurements showed that the disappearance of motor neurons was accompanied by a loss in the polyneuronal innervation of synaptic sites in the biceps muscle. The possibility that a decrease in the number of neurons contributes to the loss of polyneuronal innervation is discussed.