Objective: Although the fetus normally swallows large volumes of amniotic fluid each day, it is unclear whether amniotic fluid volume increases after fetal esophageal obstruction or whether fetal urine production changes. Our objective was to determine the effects of fetal esophageal ligation on amniotic fluid volume and urinary flow rate over time. Study Design: Seven late-gestation fetal sheep underwent esophageal ligation, and 7 served as time control animals. The urachus was ligated to eliminate urine flow to the allantoic cavity. On days 1, 3, 5, 7, and 9 after surgery, we measured the composition of amniotic fluid, fetal urine, and fetal and maternal blood, as well as amniotic fluid volume and fetal urinary flow rate. A 3-factor analysis of variance was used for statistical analysis. Results: Amniotic fluid volume did not change with time in the control group, averaging 876 ± 142 mL (mean ± SEM), and it decreased in the esophageal ligation group (P =.020), averaging 309 ± 75 mL on day 9. Fetal urinary flow rate was lower (P =.0063) in the esophageal ligation group (431 ± 27 mL/d) than in the control group (631 ± 54 mL/d). There were no differences in fetal or maternal blood compositions between the two groups. Amniotic fluid sodium and chloride increased in the ligated animals. Conclusion: Polyhydramnios did not occur after esophageal ligation, even though the fetuses excreted approximately 4000 mL of urine over the 9-day study period. This suggests that intramembranous absorption is substantially increased. With only small changes in amniotic solute concentrations, intramembranous solute absorption must occur simultaneously with water, suggesting a near-zero reflection coefficient for solutes. We speculate that fetal urine, lung secretions, or both contain a factor that increases intramembranous permeability. (Am J Obstet Gynecol 2000;182:699-705.)