The structure of monochorionic twin placentae with and without twin-twin transfusion syndrome (TTTS) was studied in mice at 17 days gestation. The TTTS donor showed anemia and growth retardation, while the TTTS recipient evidenced polycythemia and active secretion of amniotic fluid. The histological architecture of the donor placental labyrinth in TTTS showed a failure of the labyrinth to develop and immature trophoblast cells. Fetal connective tissue spaces were extensive, but no blood vessels developed there. In the TTTS recipient, the placental labyrinth developed well. Further studies are needed to establish what appears to be a promising model for human TTTS.