1. Ammocoetes were treated with iodine, potassium iodide, iodoform and thyroid extract solutions. No metamorphosis was induced.

2. Injections into ammocoetes of iodoform and of thyroid extract were made, without any metamorphic changes appearing.

3. Lamprey, ammocoete, and ox thyroids were grafted into tadpoles. Lamprey and ox thyroid were equally active in accelerating metamorphosis, but the ammocoete gland has no such effect.

4. The glands of sixty ammocoetes were analysed for iodine. They contained none.

5. The effect of iodine and thyroid on ammocoete respiration was tested. In no case was the oxygen consumption increased.

6. It is concluded that the thyroid gland has no significance in the metamorphosis of the lamprey.

I with to thank Prof. H. Munro Fox, Mr H. G. Newth and Prof. E. A. Spaul for their help.