Fetal oxygenation depends on that of the mother by the intermediary of the placenta. The respiratory feto-maternal gas exchanges are reviewed as are the regulatory factors. The greater part of the data reported here comes from experimentation of pregnant animals near term. The most important parameters for fetal oxygenation are the maternal and fetal O2 partial pressures, the affinity of the blood for oxygen, the maternal and fetal placental blood flow and the quantity of CO2 exchanged. Finally, placental efficiency as regards gas exchanges is compared to the lung.