Effect of an anti-oestrogen on implantation of mouse blastocysts

Mouse blastocysts collected on Day 4 were cultured for 4 h in Whitten's medium containing 5 micrograms CI-628 citrate, a specific oestrogen antagonist, and transferred to the uteri of pseudopregnant hosts. There was a significant reduction in the number of implantations observed on Day 11 when compared with those from blastocysts which were similarly transferred after culture in medium alone or in medium containing 1 microgram oestradiol-17 beta and 5 micrograms CI-628 citrate. We suggest that this is further evidence for the involvement of embryonic oestrogen in implantation.