2. The results may be briefly summarized as follows: (a) One injection of the gonadotropic substance of pregnancy urine produces a moderate increase in size; (b) subsequent injections of this same substance for a period of at least 3 months are without effect; (c) an alkaline extract of the whole anterior pituitary gland produces a similar increase; (d) all pituitary derivatives are ineffective in the castrated animal; (e) castration brings about a decrease in size that gradually loses velocity; (f) the male sex hormone produces a slight increase in intact, and a variable, at times conspicuous, increase in castrated animals; (g) the female sex hormone provokes a conspicuous increase in both intact and castrated animals; (h) the hormone of the corpus luteum has no effect; and (i) there is no evidence of synergism of the pituitary and male sex hormones nor of antagonism of the male and female sex hormones in adult rabbits.