1. Normal rabbits in the course of their development, after infancy, vary in their responses to minute doses of horse serum given intracutaneously.
2. There is increased reactivity in males as they mature, beginning, in the stock employed in these experiments, at about the age of 13 weeks when a weight of 1,700 gm. has been attained.
3. The reactivity of normal females remains at a constant level and the increase demonstrated in males does not take place.
4. The divergence in degree of reactivity between males and females, beginning with the establishment of estrus, indicates that factors associated with sexual maturity in females are responsible for their low reactivity.
5. The suppression of estrus by operative removal of the ovaries results in an increase of reactivity which, however, in the experiments reported here is probably not great enough to account entirely for the difference in reactivity between mature males and females.