The method of auditory evoked brainstem responses (AEBR) was applied on an animal model with two types of well defined middle ear effusions--serous otitis media (SOM) and purulent otitis media (POM). The animals with SOM exhibited fairly normal interpeak latencies but a considerably increased auditory threshold. In contrast, POM mainly affected the latencies of the brainstem evoked potential but also caused a slight increase of the auditory threshold. The results indicate that SOM chiefly causes a conductive hearing loss, while POM also affects the neural circuit.