The cellular role in the formation of amphibian enamel and dentin is under study with particular emphasis on the role of the inner dental epithelial cells which do not fully differentiate and are not enamel- forming cells. Frog's tooth germs in different stages of development are being examined at the fine structural level. Morphological data indicate that the frog's inner dental epithelial cells follow two pathways of differentiation depending on their location within the tooth germ. The formative cells adjacent to enamel differentiate into secretory cells during early amelogenesis, then pass through a transitional stage and ultimately seem to resemble transporting epithelia during late amelogenesis. The non-formative epithelial cells adjacent to dentin do not exhibit these fine structural features. The odontoblasts exhibit specialized Golgi-related structures which are related to dentin formation. Autoradiographic methods are used to define the specific roles of formative ameloblasts, nonformative ameloblasts and odontoblasts in the formation of their adjacent dental tissues. Identification of enamel and dentin precursors within their respective forming cells using high resolution autoradiography is also planned.