The fetuses released into the abdominal cavity by uterine incision escape from most physical influences of the uterus. This study examined whether these fetuses require progesterone actions for survival during late pregnancy in rats. A longitudinal incision in one uterine horn (with the other horn intact) together with bilateral ovariectomy (OVX), removal of the main progesterone-production sites, or sham OVX, were performed on day 18 of pregnancy. Thereafter the rats were given daily subcutaneous injections of anti-progesterone RU 486 (10 mg/kg), or vehicle alone, and the fetal survival rate in each uterine horn was examined on day 21. In those controls which received sham OVX plus injections of vehicle, fetal survival rates were more than 80% in both uterine horns. In the other groups, which received sham OVX plus injections of RU 486, or OVX plus injections of vehicle, or OVX plus injections of RU 486, the fetal survival rates in the intact uterine horns were 4%, 0% and 0%, respectively. In the incised uterine horns of these groups, however, the fetal survival rates were 59%, 67% and 56%, respectively. The results suggest that progesterone, which is required for maintaining pregnancy, may not be essential for survival of fetuses released into the abdominal cavity. Progesterone actions unrelated to uterine physical environment are likely to be dispensable for fetal survival during late pregnancy in rats.