To determine whether estrogen and prostaglandins have a direct effect on the preimplantation embryo, the effect of these compounds added to mouse embryos in vitro was examined. Embryos were incubated in culture media containing [3H] uridine with or without the substances to be tested. The addition of estradiol (10-8M) and prostaglandin F2a (1 pg/ml or 10,ug/ml) stimulated the incorporation of uridine into RNA. However, prostaglandin E2 at a concentration of 10 ,u g/ml had an inhibitory effect.

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... hese findings indicate that estradiol and prostaglandin F2a may have, in addition to an indirect effect mediated by uterine factors, a direct effect on embryonic growth that activates embryos to implant. [3H] uridine incorporation ; blastocyst ; prostaglandins ; estrogen ; implantation Estrogens and prostaglandins are known to play a role in the mechanism of implantation of mammalian embryos. For example, implantation occurs if estradiol is administered to animals in delayed implantation (Smith and Biggers 1968) or if prostaglandin is injected into the uterine lumen of delayed implantation mice (Holmes and Gordashko 1980). The estrogen antagonist, CI628 (Sengupta, Roy and Manchada 1981), and the inhibitor of prostaglandin synthesis, indomethacin (Biggers, Baskar and Torchiana 1981; Hurst and MacFarlane 1981) , inhibit the implantation of mouse embryos. The concentration of estradiol in serum begins to increase just before implantation in many mammalian species (Shaikh 1971; McCormick and Greenwald 1974) . Moreover, because estradiol stimulates the synthesis of prostaglandin in the endometrium (Cane and Villee