Objectives:The maintenance of endolymph homeostasis is critical for the inner ear to perform its functions of hearing and maintaining balance. The identification and cloning of aquaporins (a family of water channel proteins) has allowed the study of a novel cellular mechanism potentially involved in endolymph homeostasis. The objective of the present study was to define the developmental temporal and spatial expression pattern of aquaporin 2 (Aqp2) in the developing mouse inner ear. Study Design: A systematic immunohistochemical study of Aqp2 protein expression was performed on embryonic mouse inner ears ranging from embryonic day 10 (otocyst stage) to embryonic day 18 (just before birth). Methods: Serial cryosections of embryonic mouse inner ears were used for immunohistochemical experiments. A rabbit polyclonal antisera raised against a synthetic Aqp2 peptide was used with a standard nickel intensified 3,3-diaminobenzidine reaction protocol for immunolocalization of Aqp2 in tissue sections. Results: Aquaporin 2 is expressed diffusely in the early otocyst, then becomes progressively restricted as the inner ear matures. During early cochlear duct formation (embryonic days 12 and 13), expression of Aqp2 is homogeneous; later, it becomes restricted to specific regions of the endolymphatic compartment (embryonic days 15 and 18). Similar restriction of expression patterns could be noted for the vestibular structures. Endolymphatic duct and sac and stria vascularis expression of Aqp2 was noted to occur fairly late during development but demonstrated a distinct pattern of immunolabeling. Conclusions: Aquaporin 2 shows an early and specific pattern of expression in the developing mouse inner ear, suggesting a significant role for this water channel protein in the development of endolymph homeostasis and meriting further functional studies of Aqp2 in the inner ear.