The etiology underlying endolymphatic hydrops is scarcely understood. It is generally accepted, however, that the primary cause of endolymphatic hydrops is the malfunction and malabsorption of the endolymphatic sac. This has already been proven by animal experiments as well as by histopathological studies of human temporal bone. In this study, we attempted to induce endolymphatic hydrops by utilizing both mechanical and immunological methods. An obliterative procedure was performed on the endolymphatic sac and duct of 14 guinea pigs, 12 of which showed extensive endolymphatic hydrops after a variable period of time. The immunological method employed was to inject the antigen horseradish peroxidase into the endolymphatic sac of 22 guinea pigs that had been systemically sensitized to this antigen. Endolymphatic hydrops was induced in 11 of these subjects. Thus, this study lends further support to the fact that endolymphatic hydrops is caused by malfunction of the endolymphatic sac, resulting from either mechanical obliteration or immunological reaction.