A longitudinal sequential study of otitis media in an experimental animal (cat) using eustachian tube obstruction was done. Fifty animals were used. The continuum of mucoperiosteal changes from one day to six months after obstruction revealed gradual changes that were similar for each animal. Stages were defined, and by using different staining techniques, including immunocytochemistry, an overall middle ear defense system was postulated and documented. Nonspecific as well as specific defense systems including localized immunity, were described. Effusions were studied in a continuum, and their pathogenesis was discussed.