This study was undertaken to investigate whether the actions of ovarian hormones not related to the uterine physical state are essential for fetal survival during late pregnancy in rats. Bilateral ovariectomy and longitudinal incision in one uterine horn, which enabled exposure of the fetuses without detaching the placentas, were performed in each rat on day 14, 15, 16, 17, or 18 of pregnancy, and fetal survival was examined on day 21 or 3 days after the operation. Although almost none of the fetuses were maintained in the intact uterine horn, the fetal survival rate in the incised uterine horn increased linearly as the operation was performed during the later stage of pregnancy. In control animals, which received sham ovariectomy and the unilateral uterine incision, fetal survival rates were high in the intact and incised uterine horns. In the rats given bilateral ovariectomy and unilateral uterine incision on day 14 of pregnancy, progesterone substitution maintained fetuses in the intact and incised uterine horns examined on day 17. The results suggested that the actions of ovarian progesterone not related to the uterine physical state become less essential for fetal survival as pregnancy progresses to the late stage in rats.