Otitis media with effusion, described first by Politzer(1867), is closely related with the function of auditory tube, but its etiology and pathogenesis are not clearly defined yet. There are many theories about its pathogenesis including hydrops ex vacuo theory which is most reliable nowadays. In this paper, using cats as experimental animals, cytological study of the effusion and light microscopic observation of the middle ear mucosa in otitis media with effusion were done. The results were as follows ; 1) The effusion was found in all experimental groups after 18 hours of auditory tube obstruction. 2) In cytological study of the effusion, the fluid characteristics was not correlated with the dominant cells. The polymorpholeukocytes were dominant in earlier days but the mononuclear cells were soon increased. The macrophages were abudant on the 7th day after auditory tube occlusion. 3) In culture of the effusion, no bacteria was cultured. 4) By operating microscopy, hypertrophy of the middle ear mucosa was observed especially in the 14th day after auditory tube obstruction and effusion was most remarkable on the 14th day, also. 5) By light microscopy, there were epithelial hyperplasia, proliferation of goblet cells, capillaries and infiltration of goblet cells, capillaries and infiltration of inflammatory cells which showed same distribution as cellular analysis.