A detailed light and electron microscopic study of the development of the middle ear lining epithelium of the rat was undertaken in order to determine whether a portion of the middle ear lining is normally derived from a condensation of mesenchymal connective tissue. We learned that pharyngeal epithelium outpouches early in development and persists during the formation and growth of the ossicles and inner ear structures in a relatively constant anatomic relationship. We found no evidence of a contribution of mesodermal tissue to the lining epithelium itself. Certain features of the junctional complexes between adjacent cells were used as a means of identifying pharyngeal and middle ear epithelial cells. It is concluded that at no time is connective tissue found within the middle ear space (as defined by its epithelial limits); pneumatization occurs by absorption of mesenchymal tissue that occupied the space between the outer part of the epithelium and bone structures.