The importance of round window membrane in the pathogenesis of tympanogenic labyrinthitis or sensorineural hearing loss has been reported in the literatures. It is our intention to study a histopathological changes of round window membrane in rats following inoculation of Pseudomonas aeruginosa(P-13) at a concentration of 10⁹ organisms 1 ml(0.05 ml) into the bullar through tympanic membrane. The results were as follows : 1) Normal round window membrane in rats was composed of three layers, outer epithelial layer, middle fibrous layer, and inner endothelial layer. Any other cells showing positive PAS reaction were not found. 2) Histopathological changes in acute stage : In three days animal round window membrane was thickened, and edema, fibroblasts, hemorrhage, and mild infiltration of polymorphonuclear cells were developed. In 1 week animal infiltration of polymorphonuclear cells was increased, cells showing positive PAS reaction were found, and ciliated columnar epithelum and inflammatory polyps were developed. 3) Histopathological changes in chronic stage : Ciliated columnar epithelium was decreased from the 3 rd week and squamous epithelium and cuboidal epithelium were observed in the 6th week. Plasma cells were developed in the 2nd week and prominent from the 4th week. Gland-like structure showing positive PAS reaction was found from the 4th week. Microcyst was developed from 6th week. In general, histopathological changes of round window membrane showed a tendency to be healed from the 12 week. 4) In 1 week animal the author observed the finding that polymorphonuclear cells went into the scala tympani in basal turn of cochlea via round window membrane. The round window membrane can be penetrated more easily in early suppurative stage of the middle ear infection than in chronic healing stage of the infection. 5) Histopathological changes of round window membrane were similar to that of mucoperiosteum in round window niche. In conclusion, this model will be used in the further study of pathogenesis of tympanogenic labyrinthitis or sensorineural hearing loss as on going project.