Placental and endometrial partial pressures of oxygen (PO2) were measured using a polarographic oxygen electrode during the first trimester of pregnancy. Between 8-10 weeks' gestation, placental PO2 levels were significantly lower (P less than .001) than endometrial levels. A significant (P less than .001) increase was observed for placental PO2 values measured at 12-13 weeks compared with those obtained at 8-10 weeks. We suggest that the increase of placental PO2 at the end of the first trimester is related to the establishment of continuous maternal blood flow in the intervillous space.