Objectives/Hypothesis: It has been recognized that immunological mechanisms could be involved in the pathogenesis of different inner ear disorders, such as progressive sensorineural hearing loss, Meniere's disease, and even sudden deafness. The endolymphatic sac acts as an immune control organ for the inner ear and has been considered as an effector site of the mucosa-associated lymphoid tissue. The purpose of the study was to determine the potentially immunological relationship between endolymphatic sac and Waldeyer ring equivalent, one of the most important affector organs in mucosa-associated lymphoid tissue. Study Design: Animal model. Methods: Thirty-six rats were employed. Two animals were killed for histological observation of Waldeyer ring equivalent, and another 34 animals were randomly divided into experimental and control groups and received bilateral intranasal immunizations with keyhole limpet hemocyanin or Freund adjuvant, respectively. The ears of immunized animals and control animals were examined for keyhole limpet hemocyanin-positive memory cells and immunoglobulin G-positive plasma cells with the technique of immunohistochemical analysis. The histopathological changes and cell proliferation in those ears were also assessed. Results: There were paired and organized lymphoid tissues in the lateral wall of the first choana in the rat. Keyhole limpet hemocyanin–positive lymphocytes appeared within the endolymphatic sac at 3 days after the first anti-genetic stimulus of the Waldeyer ring equivalent. Endolymphatic hydrops in the cochlea, elevated amounts of immunocompetent cells, and increased activity of cell proliferation within the endolymphatic sac were also considered after four intranasal injections of keyhole limpet hemocyanin. Conclusion: Presumably, endolymphatic sac is supplied with immunocompetent cells from Waldeyer ring equivalent and has an ability of coreaction with Waldeyer ring equivalent. Key Words: Endolymphatic sac, mucosa-associated lymphoid tissue, proliferation.