In the experiments reported here, the fetus was removed from the amniotic cavity during the last month of pregnancy and placed in a plastic bag, with the umbilical circulation unimpaired. Its muzzle was inserted into a tightly fitting rubber glove and radioactive sodium (Na²³Cl) was added to the amniotic fluid remaining in the amniotic cavity. The maternal blood acquired a count rate significantly above the background level in less than five minutes. The lungs and the gastrointestinal system did not contain any more radioactive sodium than the other viscera. The fact that radionuclide was recovered from the fetal tissues indicates that the fetal membranes are responsible for circulation of the amniotic fluid.