Pregnant rats were ovariectomized or sham operated on day 15 postcoitum. Four days later, progesterone was measured by RIA in peripheral and uterine vein plasma, in uteri, and in placentae. Maintenance of pregnancy was not critically affected by ovariectomy, since fetal survival was 65.7 +/- 5.1% (mean +/- SEM) despite a large decrease of peripheral plasma progesterone from 115.7 +/- 3.4 to 9.3 +/- 0.5 ng/ml. Peripheral and uterine vein plasma progesterone (8.3 +/- 0.9 ng/ml) were identical. In contrast, placental progesterone decreased only slightly, although significantly, from 27.3 +/- 1.3 to 20.3 +/- 1.0 ng/mg. The concentrations of uterine progesterone were variable and positively correlated with the concentrations of peripheral plasma progesterone. It was concluded that uterine progesterone originates from peripheral blood but not from placentae and that fetal survival is positively correlated with residual progesterone concentrations in peripheral plasma and in uterus but not in placentae.