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The morphologic changes in the fetal membranes and uterine wall were studied sequentially in 8 rhesus monkeys at midgestation following replacement of amniotic fluid with 20 per cent sodium chloride. The saline solution caused a rapid stasis of the umbilical and decidual circulations as a result of dense intravascular aggregation or compaction of red blood cells. Later, small-sized blood vessels throughout the uterine wall not protected by the placenta also showed stoppage of flow. The umbilical blood flow stasis caused rapid death of the fetus from asphyxia while stoppage of the decidual circulation caused decidual necrosis. Thromboplastin release from the necrotic decidua caused a superadded intravascular coagulation with blockage of not only the remaining small blood vessels but also the entire uterine venous drainage system. The severe reductions in uterine blood flow with damage to blood vessel walls led to myometrial edema and multiple subserosal and intramural hemorrhages. After 24 to 48 hours, these combined changes led to a Couvelaire-type uterus. Uterine contractions appeared within 15 minutes of instillation of saline, and a laborlike activity evolved despite the progressive impairment of uterine perfusion and myometrial edema.