A complete or almost complete hysterectomy in the guinea pig prolongs considerably the life and function of the corpus luteum, and the latter is able to exert typical effects on the periodicity of the sexual cycle, and on the system of sex organs, especially on the mammary gland. Hysterectomy does not as such lead to a suspension of ovulation or to the proliferation of the vaginal epithelium, but it is the persistence of the corpus luteum which exerts these effects, hysterectomy acting directly only on the corpus luteum. Through hysterectomy the essential changes characteristic of pregnancy can be reproduced in the system of sex organs of the guinea pig. Some differences exist, however, between the 2 conditions, as far as the reaction of the uterus to the corpus luteum hormone and the intensity or frequency of milk production in the mammary gland are concerned. This similarity between the effects of pregnancy and of hysterectomy on the life and function of the corpus luteum suggest the possibility that also in pregnancy a functional inactivation of the uterine mucosa is responsible, at least partially, for the effects of this state on the prolongation of life and function of the corpus luteum and indirectly also for the consequences resulting from the activity of the latter organ. A partial hysterectomy leads to similar but quantitatively much less marked changes than a complete or almost complete one; even incisions into the uterine horns somewhat prolong the sexual cycle. The effects of partial extirpation concern especially the 1st sexual period following the operation, and decrease or are lost in the following ones. Differences in the effects of hysterectomy on the system of sex organs and on the periodicity of the sexual cycle in different species may at least partially depend on differences in the power of resistance to unfavorable conditions on the part of the corpus luteum as such, or of the lutein cells.