The effects of ovariectomy at different stages of early pregnancy, and the response to various amounts of exogenous progesterone and oestrogen, were studied in Q-strain mice. When the ovaries were removed on the third day of pregnancy, unimplanted blastocysts were retained in the uterus and no lysis of the zona pellucida occurred; removal on the following morning permitted lysis of the zona pellucida but again no implantation; by the afternoon the ovaries were no longer required for the initiation of implantation, but in the absence of progesterone no sites were maintained. A single dose of oestrogen eliminated all unimplanted blastocysts from the uteri of ovariectomized mice. Daily treatment with a low dose of progesterone after ovariectomy on the third day altered the appearance of the uterus but did not permit implantation unless oestrogen was also given. Under all the above conditions, some reduction in the number of embryos occurred. The maximum number of implantation sites in ovariectomized mice was achieved by daily injection of a high dose of progesterone, with no added oestrogen. An intermediate level of progesterone induced abortive implantation in mice ovariectomized on the third day, and abnormal development of implantation sites after ovariectomy on the fourth day. When the start of progesterone injections was postponed, either after ovariectomy or during lactational delay, it proved more difficult to induce implantation with the progesterone preparation alone, in the absence of added oestrogen.
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