The fine structure of the extracellular matrix during epithelio-mesenchymal interactions in the rabbit embryonic tooth germ is described using light and electron microscopy. Observations are restricted to the labial cervical loop region at the bell stage of development in maxillary and mandibular incisor tooth germs. Histochemical methods demonstrate a PAS-positive basement membrane between outer enamel epithelial cells and adjacent undifferentiated mesenchymal cells. The metachromatic region circumscribes the dental papilla region, becoming more intense and wider in association with cells illustrative of more advanced stages of differentiation. Ultrastructural observations of the basal lamina associated with outer enamel epithelial cells, cervical proliferating epithelial and mesenchymal cells, and with inner enamel epithelial cells and pre-odontoblasts are reported. The characteristic mesenchymal filopodia appeared in close proximity to the basal lamina. Microfibrils are seen as depositions on the basal lamina and in association with filopodia. The concentration of intercellular microfibrils is increased when in association with epithelia showing advanced differentiation. Collagen fibrils are frequently noted in peripheral association with mesenchymal cells but not adherent to the basal lamina. Numerous homotypic and heterotypic cell junctions are seen. An understanding of intercellular communication between heterotypic cells may be enhanced by postulating a dual developmental origin for the microfibrils associated with the basal lamina. The under surface of the basal lamina facing the extended mesenchymal filopodia appears to be a significant factor during epithelio-mesenchymal interactions, subsequent extracellular matrix formation, and morphogenesis.