Electron microscopy was used to study the characteristics of uptake of the protein tracer, horseradish peroxidase (HRP), by the middle ear mucosa of chinchillas. Two types of experimental otitis media were used: serous otitis media (SOM) produced by eustachian tube obstruction and purulent otitis media (POM) produced by inoculation of Streptococcus pneumoniae type 7F. HRP was placed in each tympanic bulla for either 1 min or 10 min. In the 1-min installation of HRP, the degree of HRP uptake was higher in SOM and POM animals than in control animals. Following the 10-min installation of HRP, the degree of HRP uptake was higher in SOM and POM animals than in control animals. Following the 10-min installation of HRP, the degree of tracer uptake was almost the same as among the control, SOM and POM animals. The mechanism of uptake was pinocytosis. In another experiment, 5-hydroxydopamine (mol. wt. 256) was used as a small tracer placed into the bulla to examine the permeability of epithelial tight junctions. No labelling of reaction product was observed in the tight junctions of the control, SOM or POM animals. These findings suggest that otitis media increases the uptake of HRP by pinocytosis and that epithelial tight junctions remain intact in inflammation of the middle ear cavity.