1. The supraspinatus tendon was traumatized in forty-six rabbit shoulders. These tendons were examined at intervals varying from forty-two to one hundred and eleven days.

2. When the shoulders were re-opened for biopsy, only five of the subdeltoid bursae had a normal appearance; nineteen showed fine adhesions; seventeen showed dense adhesions; and five were completely obliterated.

3. Some of the evidences of tendon degeneration observed in either partially or completely ruptured human supraspinatus tendons were reproduced in the supraspinatus tendons of rabbits by a single trauma.

4. The microscopic evidence of tendon degeneration was a loss of the normal wavy configuration of the collagen bundles, the homogenous appearance of the tendon, a cha ge in the character of the staining properties, an increase in the number of blood vessels observed, and the loose, oedematous, fibrillated appearance of the degenerated tendons. This fibrillation was not so marked as that observed in human tendons which had ruptured.

5. Of the forty-six tendons, 87 per cent. showed a loss of the normal wavy configuration of the collagen bundles and, instead, had a homogenous appearance, 39 per cent. showed early fibrillation, and 48 per cent. showed an increase in the number of blood vessels supplying the tendon.

6. It is concluded that the degeneration observed in the huiman supraspinatus tendon is caused by trauma incurred during the life of the individual.