Preimplantation embryos exhibit 17beta-hydroxysteroid dehydrogenase (17beta-HSD) activity which effects the interconversion of estrone and estradiol. It also changes quantitatively and qualitatively (i.e., the direction of conversion) which development, suggesting that 17beta-HSD activity may be important in regulating early embryonic development. In addition, blastocysts are capable of metabolizing progesterone. Since the oviductal and uterine function is regulated by ovarian steroids, the ability of embryos to metabolize estrogen and progesterone could affect the function of oviductal and uterine tissues around the embryos. This proposal will further elucidate the steroid-metabolizing capabilities of the embryos and their possible function in early mouse development and in embryo- uterine interaction by determining: 1. the cofactor perference of 17beta-HSD in preimplantation mouse embryos 2. the effect of NAD or NADP in medium on the 17beta-HSD activity of preimplantation mouse embryos 3. if embryonic progesterone metabolities can maintain the viability of the embryos and stimulate the deciduoma growth in the mouse uterus 4. if mouse blastocysts can produce progesterone 5. if mouse blastocysts can make estrogen 6. possible effects or functions of embryonic estrogen in mouse preimplantation development The techniques used will include: superovulation, embryo culture and transfer, histology, thin-layer chromatography, radioimmunoassay and liquid scintillation counting.