The experimental otitis media was induced by inoculation of the suspension containing pseudomonas aeruginosa (10¹¹/ml) into the bulla of healthy rats. Histopathological changes in suppurative labyrinthitis developed from experimental otitis media were observed serially with particular reference to intracochlear neoosteogenesis under the light microscope. The results were as follows : 1) Suppurative labyrinthitis was developed in 10 rats (37%) out of 27 experimental rats, 10 cases of suppurative labyrinthitis were devided into 3 stages according to the main pathology ; acute stage-3 cases, fibrous stage-6 cases, and labyrinthitis ossificans-1 case. 2) The pathologic changes were most severe in the scala tympani of basal turn in almost all animals and the endolymphatic space was relatively preserved. 3) The histopathologic findings of acute stage were observed in 3 days' animal first and also in 2 weeks' animal. The common and constant findings were infiltration of inflammatory cells (polymorphonuclear leukocytes and histiocytes) and sero-fibrinous precipitates mainly in the perilymphatic space. 4) The characteristic findings of fibrous stage were granulation tissue, infiltration of inflammatory cells (lymphocytes, plasma cells and histiocytes in order), and early stage of neoosteogenesis. 5) New bone was first found in the scala tympani of basal turn adjacent to the cochlear aqueduct in 3 weeks' animal (osteoplastic ossification). In 8 weeks' animal new bone was being formed from the fibrous tissue in the destroyed modiolus apart from the endosteum lining the perilymphatic space (metaplastic ossification). 6) In 12 weeks' animal, entire cochlear space was obliterated by new bone, of which finding was compatible with labyrinthitis ossificans.