In the inner ear, the membranous labyrinth, a tight heterogeneous sensory epithelium, separates two compartments that are filled with fluids of completely different composition. The lumen of the membranous labyrinth is filled with endolymph, a K-rich, positively polarized fluid, whereas the surrounding spaces are filled with perilymph, with a composition similar to an usual extracellular fluid. The inner ear fluids play a major role in the cochlear and vestibular physiology by the transmission of the mechanical stimulus to the hair cells, on the one hand, and by the transduction of this signal to a nerve potential, on the other hand. Numerous studies have been performed in order to know the chemical and physical characteristics of the inner ear fluids. A high, positive transepithelial potential has been evidenced in the cochlea together with a high K concentration and a low Na concentration. During the last years, the composition of the inner ear fluids, the origins of endolymph and perilymph, and the cellular mechanisms involved in the secretion of these fluids have been a great part elucidated. The present paper is a review of the contribution of the lab to the understanding of the physiology of the inner ear.