SUMMARY1. The simultaneous measurement of the two-way sodium flux across the in vitro amniotic epithelium did not reveal a preferential active movement of sodium in either direction. The amnion is thus unlikely to be the site of formation of amniotic fluid.2. The permeability of the amniotic epithelium to some non-electrolytes has been measured with isotopes and was found to be generally low in relation to the thinness of the tissue. The permeability to non-electrolytes would appear to be dependent on the oil/water partition coefficient of the permeating substance.3. The permeability to sodium and to non-electrolytes was found to vary with gestational age. The amnion from animals of 61-70 days' gestation was markedly more permeant than those at 50-60 days (term 70 days).

The authors wish to express their thanks to the personnel of the Semiscale Facilities and Test Operation Branch for their efforts in conducting the tests and collecting, verifying, and documenting the data; to the personnel of the Semiscale Data Processing Section for providing data processing support; to P. North and D. J. Hanson for their technical advice and overall contributions to improve the quality of this document; to C. P. Fineman for supplying many of the RELAP4 calculations; to K. A. Dietz for her technical review and editing contributions; and to Elinor Witbeck and Evelyn L. Pierson for their typing and proofreading efforts.

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