Adult and juvenile albino rats were maintained on a vitamin A-free test diet for 2 to 28 weeks. Histologic study of the middle ear and eustachian tube mucosa of these animals showed extensive focal squamous metaplasia when compared with the control group. Keratinization of large areas of normally columnar, ciliated epithelium was observed without frank keratoma formation. In over half of the most vitamin A deficient animals, acute otitis media with effusion was found. It is hypothesized that vitamin A deficiency disrupts the mucociliary clearing mechanism in the middle ear and eustachian tube, which may lead to effusion and otitis media.