A recently developed animal model was used to study the effect of tympanostomy tubes (TTs) on the spontaneous development of purulent otitis media. In 35 rats with soft-palate clefts a TT was inserted into the right tympanic membrane. The left ear was left intact. Serous effusion occurred in the attic space within 2 days after surgery, whether or not the middle ear cavity (MEC) was artificially ventilated. Between days 7 and 21 the intact-ear MEC was gradually filled with effusion material that turned purulent. Effusion material did not develop in the mesotympanum and hypotympanum of the intubated ears. Microbiologic examination of the effusion material showed a microflora similar to that in the nasopharynx. Ventilation through a TT reduced the number of colonized MECs (4 vs. 10) on day 21. In the individual culture-positive MEC with a TT there were fewer colonies than in the corresponding ear without a TT. These results support the contention that a TT may prevent the development of purulent otitis media.