Nidation in Progesterone-Treated, Estrogen-Deficient Hamsters, Mesocricetus auratus (Waterhouse).

In the hamster, when ovarian hormones are lacking, due to ovariectomy, or hypophysectomy, implantation does not occur and blastocysts are not recovered. 2. Implantation does not occur in hypophysectomized hamsters with pituitary autografts. 3. Doses of progesterone, 2-4 mg daily, do not affect time of implantation in the intact hamster. 4. Implantation occurs with no apparent delay in hamsters made estrogen-deficient by ovariectomy, ovariectomy and adrenalectomy, or hypophysectomy and given daily injections of 2-4 mg progesterone. In experiments designed to eliminate estrogen in injection media and diet, implantation also occurred when progesterone was given to ovariectomized hamsters. This suggests that progesterone alone is required for maintenance of blastocyst and nidation in the hamster.