In the epithelium lining the nasopalatine duct of the infant mouse, a transitional zone between the stratified squamous epithelium and the ciliated columnar one can be observed. The epithelium lining the transitional zone shows gradations ranging from the stratified squamous through the stratified cuboidal to the ciliated stratified low-columnar type, and gradually transforms into the stratified squamous epithelium with advancing ages. In the adult mouse, the nasopalatine duct is lined with the stratified squamous epithelium throughout up to the vicinity of the nasal cavity, and changes abruptly into the ciliated columnar epithelium lining the nasal cavity. It is suggested that the epithelium lining the transitional zone is identical with the ‘intermediate epithelium’ in the mouse nasopharynx and epiglot