1. The presence of stretch receptors in the muscles of Raja and Scyllium has been physiologically demonstrated. Their morphology is unknown.

2. The behaviour of a single end-organ in the radial muscles of the pelvic fin of Raja clavata has been examined in detail.

3. The adaptation curve and the tension-frequency relationship have been determined.

4. Under constant tension a single end-organ has continued to function rhythmically for over 1 hour.

5. When tension is suddenly reduced, there follows a silent period before the frequency of discharge corresponding to the reduced tension appears.

6. The quantitative aspects of the behaviour of the stretch receptor of Raja bear a closer resemblance to those of the receptors of the carotid sinus than to those of mammalian and amphibian limb muscles.

7. The response of muscle receptors during a passive undulatory movement of the pectoral fin has been recorded, and their role in the swimming rhythm is discussed.

Rockefeller Foundation Fellow.