The ultrastructure of the amniotic epithelial cells from normal, polyhydramnic, and oligohydramnic pregnancies were studied with the transmission electron microscope. The amniotic epithelial cell layer from normal pregnancies was 8- to 12-μ thick. In polyhydramnic pregnancies the cell layer varied widely from the normal thickness to as much as 18 to 56 μ in diabetic patients. Other ultrastructural changes observed in pregnancies complicated by polyhydramnios were abnormal microvilli, diminished intercellular canals, increased tonofilaments, and decreased rough-surfaced endoplasmic reticulum. In pregnancies complicated by oligohydramnios the thickness of the cell layer was 3 to 6 μ. The amniotic epithelial cells had increased tonofilaments, decreased desmosomes, collapsed and fused intercellular canals, and caused a marked decrease of the Golgi apparatus and the rough-surfaced endoplasmic reticulum. The sparse microvilli were short, plump, and had bizarre shapes.