Tissue blocks removed from placentas of rats and mice at different gestational age were fixed in either OsO4 or glutaraldehyde or both, embedded into Araldite, and thin sections were examined with the electron microscope. A hitherto undescribed cytoplasmic component consisting of convoluted thread-like elements embedded in a clear ground substance has been regularly found in the trophoblast II cells of rats and mice. The ovoid-shaped cytoplasmic component, termed by us glomerular body, is about 1 to 1.5 μ in its greatest diameter and is composed of electrondense 400 to 500 Å thick threads which appear to have a fine filamentous and granular structure. The threads were often observed in close topographical relationship with free ribosomes. The thread-like elements of the glomerular body were occasionally found dispersed or lined up under the cell membrane of the foetal side of the trophoblast II cell. In some instances similar structures were observed in the nucleus. The nature and significance of the glomerular body is unknown.