We have shown, through a test proceeding from the McPhail test, in the immature rabbit female, the existence of succession and simultaneity synergisms between ethynylestradiol and pure or impure norethisterone, given by oral route. The estrogen, administrated before the progestative, is able to potentialize the action of the latter on the building of the endometrial lace in 5 days, more than on the lengthening of it in 15 days. Ethynylestradiol, absorbed with norethisterone, strengthens the effects of the latter on the second phenomenon more than on the first one. Absolute and relative doses of the estrogen and of the progestative differ only lightly according as norethisterone is pure or not pure, because of the more potent estrogenicity of the impure norethisterone.