The enamel organ of rat molars has been examined with light and electron microscopy. At the embryonic age of 20 1 2 days M 1 and M 2 are in the bell stage of development. No deposition of inorganic material has taken place. The cytoplasm in the cell population of the enamel organ is characterized by the presence of large numbers of free RNP particles. The Golgi apparatus is represented by vesicles, tubules and flattened cisternae. Many capillaries are present on the surface of the outer dental epithelium. Vascularization of the enamel organ via capillary penetration into the stellate reticulum has not occurred. A basement membrane is seen surrounding the entire enamel organ. The outer dental epithelium cell surface bordering this basement membrane has areas apparently specialized for attachment purposes. These areas have been termed hemi-desmosomes. Typical desmosomes are seen throughout the enamel organ but most frequently in the stratum intermedium and in the stellate reticulum. Glycogen and lipid material have been identified in the electron microscope sections. Glycogen deposits occur in the form of particles 150–400 Å in diameter and are most often seen in the cells of the outer dental epithelium. The lipid particles are generally amorphous, varying in size up to 0.3 μ in diameter and are most often found in the outer dental epithelium.