The steroids secreted by the embryo have been postulated to play important roles in embryonic development and in implantation interactions between the embryo and the endometrium. Recently we have found (a) both rat and mouse blastocysts exhibited high levels of 17Beta-hydroxysteroid dehydrogenase (17Beta-HSD) activities and (b) testosterone was metabolized into several metabolites by rat, mouse, and hamster blastocysts with marked species differences. This proposal is designed to extend and broaden these findings, aiming at defining the steroidogenic capacity of the preimplantation embryo in the mammals, so that the functional significance of embryonic steroids can be accurately assessed. We plan to employ radioimmunoassay, embryo culture, thin-layer chromatography and recrystallization to constant specific activity in the following studies: (1) Effects of estradiol concentrations on 17Beta-HSD activities in mouse embryos. (2) Ontogeny of 17Beta-HSD activities in the mammalian embryos. (3) Effects of the antiestrogen CI-628 and 17Beta-HSD activities in mouse embryos. (4) Effects of histamine and prostaglandins on 17Beta-HSD activities in mouse embryos. (5) Metabolism of testosterone by preimplantation mouse embryos and isolation and identification of its major metabolites. (6) Effect of histamine, prostaglandins, and gonadotropins on estradiol production from testosterone in rabbit balstocysts.