In previous studies (Dickmann & Dey, 1973, 1974; Dey & Dickmann, 1974) we demonstrated Δ5-3β-hydroxysteroid dehydrogenase (HSD) activity in rat morulae and blastocysts. Since HSD activity in a tissue is strong evidence for steroidogenesis (Wiest & Kidwell, 1969), we assumed that morulae and blastocysts synthesize steroid hormones. Based on these and other findings, we postulated that steroids produced by the embryo play an important, perhaps decisive, role in the phenomena of morula to blastocyst transformation and implantation of the blastocyst. The present study explored three factors which may regulate the HSD activity (i.e. steroidogenesis) in rat blastocysts: the uterine environment, pituitary hormones, and ovarian hormones. Adult virgin female rats of the Holtzman strain, weighing 180–220 g, were mated with fertile males. The morning spermatozoa were found in the vagina was designated Day 1 of pregnancy. At the end of the various experiments, the rats were killed and blastocysts