Aquaporin 4 (Aqp4), a member of a family of water transport proteins, is a candidate for playing a critical role in inner ear fluid homeostasis. In this study, we assess cross-species Aqp4 expression in the inner ear, sequence conservation in the 5'-UTR, and hearing in Aqp4 knockout mice. A single Aqp4 isoform was expressed in a highly conserved pattern within the supporting epithelia surrounding the sensory cells of the auditory and vestibular sensory organs and the glial cells surrounding the auditory nerve of the mouse and rat. In the 5'-UTR of mouse and rat Aqp4 gene, sequence conservation was highest in the region spanning the transcription start site. Aqp4 knockout mice demonstrated impaired hearing, but normal neural conduction time. Similar Aqp4 expression pattern and regulatory sequence conservation across species suggest a highly conserved role for Aqp4 in the inner ear. In the Aqp4 deficient mouse, cochlear dysfunction is suggested as the primary cause of hearing impairment in the absence of neural conduction abnormality. (C) 2002 Elsevier Science (USA). All rights reserved.