1. Ten pregnant ewes were operated on at 130 days of gestation. The fetal trachea was intubated with a double‐lumen tube, an inflatable occluder was placed around the umbilical cord, vascular catheters were placed in the fetal carotid artery and jugular vein and in the maternal jugular vein, and multiple catheters were placed in the amniotic and allantoic sacs. 2. At 139 days gestation, the fetus was ventilated in utero, and the umbilical cord was occluded. The extrafetal fluids were circulated by means of roller pumps. Known activities of radio‐iodinated human serum albumin, tritium‐labelled water and 14C‐labelled urea were injected into the amniotic and/or allantoic fluids. Samples were obtained at 30 min intervals for several hours. 3. Extrafetal fluid volumes were calculated from the albumin distribution volumes. The amounts of labelled water transferred to the maternal circulation were calculated from the changes in tracer concentrations in extrafetal fluids and fetal plasma. 4. No labelled albumin was detected in fetal or maternal plasma. The permeability‐surface area product of labelled water at the combined amniotic and allantoic interfaces with the ewe was 28.2 +/‐ 2.8 ml/min (mean +/‐ S.E.M.). In five preparations the values could be separately calculated for amniotic and allantoic interfaces. The two mean values (19 +/‐ 4 and 12 +/‐ 1 ml/min) were not significantly different from each other. The permeability‐surface area product at the combined interfaces with the fetus was 0.96 +/‐ 0.17 ml/min. Urea was so much less permeable than water that no reliable permeability‐surface area products could be calculated in all of the preparations. 5. We calculated that the hydraulic conductivity of the combined extraplacental pathway is more than 0.5% of that of the placenta. Because the osmotic gradient across the extraplacental pathway is one to two orders of magnitude greater than that across the placenta, extraplacental transfer of water can significantly affect intrauterine water volume.
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