Experimental otitis media was produced in chinchillas by eustachian tube obstruction or pneumococcal infection. Sequential changes in the histology of the middle ear mucosa and enzyme profile of the middle ear effusions (MEE) were studied. In serous otitis media (SOM) which followed tubal obstruction, the subepithelial space was widened by edema and capillary dilatation, and the middle ear space was filled with serous fluid. Slight hyperplasia of epithelial cells was also observed. The subepithelial space remained widened with mild fibrous change and capillary dilatation, and slight hyperplasia of epithelial cells persisted 42 days after obstruction. In purulent otitis media (POM), which followed inoculation of pneumococci into the middle ears, metaplasia of the epithelial layer from flat to columnar cells was observed. The subepithelial space was widened with loose fibrous connective tissue proliferation, vascular dilatation and inflammatory cell infiltration. Both lactate dehydrogenase (LDH) and lysozyme levels in MEE were higher in the POM group than in the SOM group. When bacterial enzymes, hyaluronidase and lipase activity were measured in MEE and plotted together with the percentage of positive culture of the MEE at different times after the experimental infection, the enzyme activities decreased with the clearing of bacteria and along with the resorption of inflammatory changes of middle ear mucosa evidenced by histology. In human MEE studies, immunoglobulins (IgG, IgA, IgM) of MEE were higher than in serum except IgM in serous MEE. The IgG content of MEE in the culture-negative group was higher than in the culture-positive group. Possible mechanisms for this difference were discussed.