Serous otitis media is closely related with auditory tube function, but its etiology and pathogenesis are not clearly defined yet. So we tried to prove the theory of hydrops ex vacuo via the experimental study with cats by means of obstructing the pharyngeal orfice of the auditory tube and observe the serial changes in tympanic mucosa through light and electron microscopy. The results are as follows; 1. We confirmed the production of serous otitis media with auditory tube obstruction and have a new understanding of auditory tube function in middle ear aeration. 2. The effusion in serous otitis media was produced from the next day of experiment and increased till the fourteenth day, but decreased after the spontaneous perforation of ear drum. 3. Through the light microscopy, we observed the increasement of the sercretory cells including goblet cells, epithelial hyperplasia, capillary proliferation and invasion of inflammatory cells. 4. Through the elctron microscopy, we observed the protrusion of secretory cells, blobs in cilia, loss of cilia, increasement of vesicles, vacuoles and dense bodies in ciliated cells and Invasion of inflammatory cells. With above results, we concluded that aeration through auditory tube is the most important factor in serous otitis media and presumed the effusion was secreted by secretory cells.