Serial measurements of amniotic fluid volume were made using a tracer dilution technique in sheep during the last half of gestation. No consistent trends were observed in amniotic fluid volume changes over this period. Disappearance of tracer-labeled urea and water from the amniotic fluid was observed at various gestational ages during the last half of pregnancy. The rate of disappearance of urea from the amniotic fluid decreased with gestational age. The disappearance rate of water did not change. Relationships between disappearance rates and amniotic fluid volume suggest that the permeability of the fetal membranes may be important in determining amniotic fluid volume.