Several facts have suggested that the gonadotrophic potency of the pituitary may be diminished by progesterone. Loeb [1914] noticed that extirpation of the corpora lutea in the guinea-pig was followed by an earlier onset of the next ovulation, and that this occurred even if the animal were in the early stage of pregnancy. Moreover, an artificially caused extra-uterine pregnancy, which led to degeneration of the corpora lutea, was accompanied by ovulation in spite of the presence of a living embryo within the abdomen. These phenomena might be explained by supposing that the luteal hormone progestin inhibits ovulation by its action on the pituitary. Papanicolaou [1920], also using the guinea-pig, found that removal of the corpora lutea within 24 hours after oestrus and ovulation accelerated the onset of the next oestrum, the interval between the successive oestra being reduced to 11 days instead of the normal 16–17. Removal of the corpora