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The amount of gonadotropin and estrogen passing from tlie maternal to the fetal organism may be gauged by comparing the levels of these substances in the blood of the maternal organism and in the vein of the umbilical cord. The concentrations of horniones in the blood of the mother and the uinbilical cord may, therefore, serve as a criterion of the permeability of the placenta to these horniones.The estrogen.'* 'as well as the gonadotropin'. concentration in the blood of the unibilical cord has repeatedly been studied but without siniultaiieous hornione determinations in the maternal blood.In the following experiments, the estrogen and gonadotropin concentration of the umbilical vein, on the one hand, and that of the retro-placental seruni, on the other, were quantitatively studied.Immediately after cutting the unihilical cord the arteries were clamped and the fetal blood flowing from the unibilical vein (about 20 cc) collected in a test tube. The retro-placental (maternal ) blood was obtained during the expulsioii of the placenta. After coagulation, the serum of both specimens was centrifuged.The sera were 3 tinies vigorously shaken with ether. The small amounts of ether which remain in the seruni were i-enioved by allowing the sera to stand in the open air for an hour. These sera contained no free estrogen, as showii by control determinations in castrated female mice, and each were tested on 8 infantile female albino rats for their gonadotropin content. The quantity of seruni to be tested was given to the animals in 6 subcutaneous injections over a period of 48 hours. From the 60th to the 120th hour after the experiment was liegun, vaginal smears were taken; after this tinie, the animals were sacrificed and uteri and ovaries inspected. We considered 1 RU as that quantity of hornione which, with tlie above technic, produced full vaginal Experimentnl.Detcrriziitation of Gonndotropiii.