The effect of fluoxymesterone (FMA), a fluorinated androgen, on ovum implantation has been studied using ovariectomized, progesterone-treated pregnant (‘delayed’) mice. 500 µg FMA, given either continuously from day 4 to 9 or singly on day 8, was able to induce implantation but at the same time caused total resorption of implanted embryos in the case of prolonged treatment with the drug. This anti-estrogenic activity of FMA could be reversed by supplementation of 0.25 µg estradiol-17 β given on day 8. Testosterone also induced implantations in ‘delayed’ mice. FMA and testosterone given to long-term ovariectomized, progesterone-primed mice showed characteristic changes in uterine epithelial cell morphology similar to the attachment found in normal pregnancy. The possible mechanism of action of FMA in early pregnancy will be discussed.