The prenatal development of thoracic spinal cord was studied by electron microscope in human embryos and fetuses ranging from 9mm to 260mm crown-rump length (5-30 weeks of gestational age). Ependymal cells in all fetal ages had conspicuous junctional complexes close to the lumen of the central canal into which microvilli and cilia projected. The ependymal cells contained numerous longitudinally arranged mitochondria, flattened cisternae of endoplasmic reticulum and Golgi complex. At 20 mm embryo, the floor and roof plates were composed of ependymoglial cells and undifferentiated neuroepithelial cells. The neuroepithelia of the sacral spinal cord were delineated from central medullary cord. By 100 mm fetus few undifferentiated neuroepithelial cells remained in the floor and roof plates. At 150 mm fetus, the whole central canal was formed by ciliated columnar epithelial cells containing cilia with basal bodies. The microvilli became tangled and club-shaped and formed a matted surface. The canal was filled with areas of dark and pale amorphous materials bounded by membrane-like structure. These two types of material were found throughout the whole central canal from 100 mm fetus onwards. By 260 mm fetus, microfibrils were first observed in the ependymal cells. In conclusion, it seems that early development and differentiation of central canal ependyma are simlar to that in other part of the brain ventricular system although ependymoglial cells are more prominent.