Centchroman was administered to rats in relation to luteal phase nidatory estrogen secreted between 21.00 on day 4 and 10.00 on day 5 of pregnancy. A single oral dose of 1.25 mg/kg before the secretion of nidatory estrogen, i.e. until 21.00 on day 4, prevented implantation in 100% of the rats without altering plasma estradiol or progesterone concentration. Administration of a dose of up to 62.5 mg/kg at 10.00 on day 5 even failed to inhibit implantation, but caused dose-dependent resorption of implantations. Resorption of all implantations at a dose of 62.5 mg/kg was associated with a decrease in circulating progesterone levels, but was only partially reversed by progesterone or progesterone + estrone supplementation. Apparently normal morulae and blastocysts recovered between days 4 and 10 from rats treated with anti-implantation doses before release of nidatory estrogen, when transferred to the uteri of control rats exhibited lower pregnancy, implantation and development rates with increasing confinement in the genital tract of treated donors. None of the embryos recovered from control or centchroman treated females implanted in the uteri of treated rats. Centchroman administration on day 1, but not on day 5, abolished endometrial receptivity to an artificial stimulus for decidualization. All term fetuses were apparently normal and their weight comparable to that of control fetuses. The study provides evidence of post-implantation fetal resorption, occurring primarily between days 10 and 19 post-coitum, in rats treated with a potent antiestrogen before or immediately after the prenidatory estrogen secretion. This was not related to altered ovarian function and the decreased plasma progesterone concentration on day 19 after a higher dose of the compound was secondary to resorption of all implantations.