Evidence that the maintenance of early pregnancy in the rabbit requires ""blastocyst estrogen''

It has been shown by others that treatment with only progesterone is sufficient to induce implantation and maintain pregnancy in ovariectomized-adrenalectomized rabbits. Based on these results, it was deduced that induction of implantation in the rabbit does not require estrogen. In making this deduction, ""blastocyst estrogen'' was not taken into account. In the present study, blastocyst development and implantation were prevented by instilling the anti-estrogen CI-628 into the uterine lumina of pregnant, ovariectomized, progesterone-treated rabbits. Our interpretation of these results is that the CI-628 PREVENTED THE ACTION OF ""BLASTOCYST ESTROGEN''. However, on the basis of the present results, we cannot determine whether the specific target of ""blastocyst estrogen'' is the blastocyst, the uterus, or both.