I . Sexual maturity in lampreys is accompanied by modifications of the cloacal region. Thc cloacal labia become swollen by an infusion of blood and "ducts" form, which lead from the coelom to the mesonephric ducts. These "ducts" are not present in the immature adults or ammocoete larvae.
2. These changes may be produced in the immature adults and ammocoete larvae, by injections of a mammalian-pituitary preparation. In the ammocoetes the response to even a small dose was very rapid, ducts being formed in many cases twenty-four hours after the injection of I mg. of A.P. 15 B.
3 . Testosterone and oestrone also produced these modifications in the adult, but not in the larva.
4. Spermatogenesis of the adult was slightly affected by gonadial hormones and pituitary preparations.
5 . The gonads of the larvae were not affected by gonadial hormones or pituitary preparations in the doses used.
6. The rapidity of the cloacal response to the injections of pituitary preparations in ammocoetes suggests that this response is of a direct nature. The absence of changes in the gonads of injected ammocoetes supports this view.