Abstract A new hypothesis is proposed concerning the hormonal control of blastocyst implantation. In preparation for implantation, the uterus has first to come under the dominance of progesterone, derived from the ovaries. As the time of implantation approaches, a distinction is made between the conditioning of the prospective implantation sites and the remainder of the uterus. While progesterone-dominance has to be maintained in the interimplantation areas, the implantation sites have to be relieved temporarily from the dominance of this hormone (an anti-inflammatory agent), in order to permit a local inflammatory-like reaction which is a prerequisite for implantation. The local relief from progesteronedominance is accomplished through the action of oestrogen released by the blastocyst. In some species, like rat and mouse, blastocyst oestrogen by itself is inadequate to overcome the progesteronedominance. Therefore, the action of systemic oestrogen (the “oestrogen surge”) has to precede the action of blastocyst oestrogen.