1 . The extensor digitorum longus of the rabbit was partly denervated by section of one of its two nerve branches and examined histologically for evidence of sprouting of new fibres.

2. Sections from material fixed two and three days after operation showed terminal bundles in which varying numbers of axons and motor end-plates have degenerated. This supports the concept that the motor unit is not confined to single groups of neighbouring muscle fibres, but innervates fibres scattered throughout the muscle.

3. New fine fibres branching from intact intramuscular axons to reinnervate denervated muscle fibres were observed as early as four days after operation.

4. Such new fibres were most numerous in the early weeks after operation and their numbers then declined. Two months after operation no small fibres or simple end-plates were seen.

5. No new fibres were seen in areas of the muscle containing only denervated nerve fibres. The new fibres were formed only under the stimulus of proximity to the degenerating ones.

6. The relationship of these findings to the mechanism of recovery of human muscle affected by poliomyelitis is discussed.