Under an experimental condition we found decapitation of fetuses in utero prevented the onset of parturition in the rat. Injection of ACTH into the decapitated fetuses resulted in parturition of these fetuses. The result indicates that the fetal pituitary-adrenal axis is involved in the mechanism by which parturition is initiated and agrees with the results reported for the sheep and the goat. This proposal is to study further the mechanism involved in the onset of parturition in the rat with special respect to the role of ACTH in the fetus. We will (1) study the secretory pattern of corticosterone, progesterone and estrogens by the feto-placental units before and after ACTH administration to near-term fetuses (2) investigate if the alternation of steroid production by the fetoplacental units may affect prostaglandin production in the uterus, (3) attempt to suppress the ACTH secretion by the fetal pituitary and examine the effect on the time of parturition and (4) demonstrate that the fetal-adrenal axis is involved in determining the time of parturition in superimplantation rats.