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1. Na transport has been studied in the isolated rat visceral yolk sac from day 17.5 of gestation to term. 2. The transepithelial potential difference (p.d.) and the short circuit current (s.c.c.) in the isolated yolk sac were found to vary with gestational age, with peak values at day 19.5. The maximal p.d. and s.c.c. were 3.85 +/- 0.32 mV (the fetal side positive) and 19.5 +/- 5.3 microamperemeter cm-2 respectively. 3. Simultaneous determination of the two-way Na+ flux and the s.c.c. revealed a preferential active movement of Na in the maternal to fetal direction. The net flux was found to be 50% higher than the s.c.c. 4. Both the p.d. and the s.c.c. were found to be reduced by cooling and by the uncoupling agent 2,4-dinitrophenol. 5. The s.c.c. altered in a curvilinear fashion with the Na+ concentration in the bathing solution, with an apparent Km of about 20 mM-Na+. Removing Cl ions from the bathing solutions had no effect on the p.d. and s.c.c. 6. Addition of amiloride (10(-4) M) to either side of the visceral yolk sac had no effect on the s.c.c. but application of ouabain (10(-5) M) to the fetal side caused a profound fall in the s.c.c. 7. The possible physiological role of this active Na transport by the visceral yolk sac in the formation of amniotic fluid is discussed.

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