There is increasing evidence that implicates reduced amnionic fluid volume as a major determinant of fetal risk in prolonged pregnancy. We sought to determine whether reduced fetal urine production might be associated with oligohydramnios in pregnancies that reach 42 weeks or more. Ultrasonographic measurements of the fetal bladder were obtained every 2 to 5 minutes for 1 hour in 38 gestations verified to be at least 42 weeks. Oligohydramnios was present in eight of the prolonged pregnancies. Similar measurements were performed in 15 normal pregnancies delivered by elective repeat cesarean section between 38 and 40 weeks' gestation. Hourly fetal urine production rates were calculated with sequential bladder volume measurements. The result of this investigation suggest that diminished fetal urine production is associated with oligohydramnios in prolonged pregnancy. The mechanism by which fetal urine production is reduced in prolonged pregnancy remains unknown. A likely possibility is reduced fetal swallowing because of already diminished amnionic fluid volume, the latter a result of placental senescence.