The anti-implantation action of tamoxifen in mice

The process of implantation of the blastocyst in utero in the mouse is normally triggered by oestradiol-17 beta. It can, however, be prevented by the drug tamoxifen whose actions in other species are classified as anti-oestrogenic, but which the mouse is held to be a weak oestrogen. Results are presented which support the claim that the anti-implantation action of tamoxifen is dose dependent, oestrogenicity dominating at high doses and anti-oestrogenicity being of major importance at lower doses.