Experimentally lipopolysaccharides(LPS) of H. influenzae and K. pneumoniae which are injected in the middle ear induced inflammatory changes of the middle ear mucosa and otitis media with effusion(OME) for short time. The purpose of this study was to investigate the morphologic changes in the middle ear and Eustachian tube mucosa after the transtympanic injection of the LPS from E. coli and whether the persistent endotoxin, such as experimentally repeated injection, could produce the chronic OME or not. 48 Guinea pigs were used. Group A, using one injection of saline solution containing 50μg/ml of LPS from E. coil into the tympanic cavity, and group B, using three injection of 25μg/ml LPS 4days interval, were seperated. Morphologic studies were made by the light microscopy and transmission electron microscopy. Repeated exposure to LPS produced thicker effusion, resulting in the development of long term OME. On cytologic analysis of effusion, the cells were exclusively polymorphonuclear leukocytes at the days 2 and 4 on group A, but polymorphonuclear leukocytes, macrophages and lymphocytes were found at the days 2, 4 and 7 on group B. Ciliary abnormalities and high secretory activity were observed in the middle ear mucosa adjacent to Eustachian tube, but there was no significant increase in the number of secretory cells on TEM. In conclusion, repeated injection of lipopolysaccharides produced more, 1) prolonged thick effusion, 2) prolonged secretory activity, 3) severe inflammatory changes in the middle ear and Eustachian tube than single injection. Therefore, the persistent endotoxin may be assoaiated with chronic mucosal pathology.