1. The sensory functions of the inner ear (hearing and balance) critically depend on the precise regulation of two fluid compartments of highly desparate ion composition, i.e., the endolymph and the perilymph. 2. The parameters volume, ion composition, and pH need to be held at homeostasis irrespective of the hydration status of the total organism. 3. Specific cellular water channels, aquaporins, have been shown to be essential for the fluid regulation of several organs, e.g., kidney, lung, and brain. 4. Because of functional similarities of water regulation in the kidney and inner ear this review initially summarizes some aquaporin functions in the kidney and then focuses on 6 out of 11 mammalian aquaporins that are present in the inner ear (AQP1-6). 5. Their potential role in the inner ear fluid control will be discussed on the basis of the respective expression patterns and individual pore properties. 6. Further, a working model is presented of how the endolymphatic sac may contribute to inner ear fluid regulation.