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With a view to clarifying the disease state of exudative inflammation of the middle ear, the pressent study was conducted to look for underlying problems by way of clinical statistics and to make observation of changes of the middle ear mucosa in experimental animals.The condition has generally been ragarded as a clinical entity classified among the diseases of the eustachian tube and attributed to tubal obstruction. A retrospective study of patients with exudative otitis media, however, showed that the ear canal on the affected side was frequently not impeded in chronic cases of unilateral involvement where, therefore, the condition could be hardly ascribed to tubal obstruction but rather due to disorders of the middle ear. In view of this, an attempt was made to investigate mucosal changes of the middle ear in guinea pigs with experimentally induced exudative inflammation of the middle ear without direct obstruction of the ear canal. Injection of viscous fluids into the tympanic cavity or reduction of pressure in the middle ear was found to result in retention of fluid in the middle ear cavity. The mucosa was observed to undergo dysplastic changes into mucosal epithelium with secretory cells as a result of inflammation or in response to stimulation of the middle ear.It would follow that exudative otitis media is caused (1) by obstruction of the eustachian tube or (2) from pathologic changes in the middle ear, and the data suggest that the condition which has arisen from tubal obstruction may turn in its progress into that form attributable to pathologic changes of the middle ear.