OBJECTIVE Recently an intramembranous pathway was reported in the ovine fetus as a route for the rapid exchange of water, ions, and molecules between the amniotic fluid and the fetal blood that perfuses the fetal surface of the placenta and the fetal membranes. Our study was designed to test the hypothesis that the amniotic fluid composition would gradually equilibrate with fetal plasma when the major flows to and from the amniotic compartment were eliminated. METHODS Eleven near-term fetal sheep underwent ligation of the urachus to eliminate the allantoic fluid. An inflatable cuff was placed around the esophagus and trachea, and catheters were placed in the fetal urinary bladder, fetal circulation, and maternal circulation. At > or = 5 days after surgery the animals were subjected to either a control experiment or a continuous urine drainage plus tracheoesophageal occlusion for 8 hours. RESULTS During the urine drainage plus occlusion study, amniotic fluid osmolality (p < 0.0001), Na+ (p < 0.0001), K+ (p < 0.01) Cl- (p < 0.001), and lactate (p < 0.001) increased compared with the control experiment. These corresponded to 50% reductions in the gradients for osmolality and Na+ between fetal plasma and amniotic fluid; the K+ gradient increased, and the Cl- gradient reversed. The percentage increases in amniotic Na+, K+, Cl-, and lactate were all 10% at 8 hours. CONCLUSIONS These observations suggest that water is absorbed from the amniotic fluid through the intramembranous pathway into the fetal circulation at a rate of 1.25% of the total amniotic volume per hour or approximately 240 ml/day.