Acute experimental serous otitis media was induced in the squirrel monkey following Eustachian tubal obstruction. A multidisciplinary study of the mucoperiosteum included light microscopy, enzyme histochemistry, electron microscopy, biochemistry and capillary permeability. Oxidative and hydrolytic enzyme activity was generally increased throughout the mucoperiosteum. The periosteum showed a strong positive reaction for alkaline phosphatase. In this model the passive transfer of serous fluid from subepithelial vessels to the middle ear was suggested by 1) chemical similarities of serous effusion and blood serum, 2) retention of carbon particles within the subepithelial small vessels in the presence of serous otitis media, and 3) ultrastructural evidence of basement membrane rupture and fluid flooding intercellular spaces, thereby distorting epithelial cells.