Certain facets of pregnancy in gonadotropin—stimulated prepubertal rats have been compared with pregnancy in adult rats. Minced tissues of embryonic implantation sites of the 13th day and fetuses and placentas of the 16th day of pregnancy metabolized progesterone-4-14C to 5α- 3,20-dione which was then converted to 3α and 3β hydroxy derivatives by both placenta and fetus. The reduction of progesterone by fetus and placenta may be one of the major factors in decreasing the concentration of plasma progesterone in the latter half of pregnancy in the rat. Experiments designed to elucidate the physiologic role of these progesterone metabolites showed that 5αdihydroprogesterone retains no progestational properties, as measured by the decidual cell reaction, but caused slight increase in the uterine wt, whereas the 5β isomer and the 3α and 3β hydroxy epimers did not. The survival of blastocysts in gonadotropin—stimulated prepubertal rats spayed on day 3 of pregnancy was tested. Both progesterone and 5α-pregnane-3,20-dione promoted the viability of the blastocysts and led to the survival of a larger number of blastocysts than in the animals treated with sesame oil. However, only progesterone and not 5α-pregnane-3,20-dione could, in conjunction with estradiol-17β, initiate implantation and maintain gestation. (Endocrinology92: 83,1973)