Summary. Zygotes recovered from the oviducts and uteri of mice treated with the anti-oestrogens dms, MRL 37 or MER 25 implanted normally after transfer to pseudopregnant host mice, but treatment of the hosts with an anti-oestrogen inhibited implantation. Induction of deciduomata in intact and ovariectomized mice was prevented by prior treatment with oestradiol or one of the anti-oestrogens, although MER 25 and MRL 37 were less effective in ovariectomized mice. When implantation was delayed by ovariectomy, MRL 37 and MER 25 largely prevented oestradiol-induced implantation on Day 8 without affecting the recovery of blastocysts, but dms induced implantation in some mice and did not significantly affect the induction of implantation by oestradiol. dms, and to a lesser extent MRL 37, caused oestrogenic changes in the endometrium on Day 4. It is concluded that the antifertility actions of dms are due to oestrogenicity, but those of MER 25 and MRL 37 may be due to their anti-oestrogenic properties.