1. The effects of injection of (a) Schockaert's extract of anterior pituitary, (b) Van Dyke & Wallen-Lawrence's phyone and (c) prolactin, upon the growth of larval neotenic Amblystoma tigrinum, have been investigated. The normal growthrate was established before injections were begun and controls were injected with muscle extract.
2. The dosage adopted in all cases was equivalent to 1 mg. of fresh tissue per gram of axolotl per day. This was probably supramaximal.
3. Schockaert's extract and phyone caused a marked increase in the rate of growth, prolactin a slight increase.
4. Of the ten animals injected with Schockaert's extract, all except one showed some of the signs usually associated with metamorphosis in A. tigrinum. The other experimental animals did not.
5. In no case were there any abnormal changes in the bodily proportions or in the shape of the skeleton.
6. The growth of half the animals after injections had been stopped was also investigated. In the case of most of the animals injected with Schockaert's extract or with phyone, there was a loss of weight, followed by recovery and growth in the case of the latter only. The prolactin-injected animals showed no such reaction.