1. Three cases of paralysis of the radial nerve after intensive muscular effort in extension of the elbow are described. Clinical and electrophysiological evidence is recorded.

2. There was motor and sensory affection of varying degree and extent in the distribution of the radial nerve.

3. The type of lesion in each case was that of neurapraxia, and rapid spontaneous recovery occurred in each case.

4. The level of the lesion was in the arm, below the origin of the branches to the triceps.

5. The cause of the lesion was thought to be compression by a fibrous arch related to the lateral head of the triceps.

6. The cases are discussed in relation to other instances of compression of nerves by fibrous arches, and an explanation is advanced for spontaneous recovery.