The internal environment of multicellular organisms is split into specialized compartments containing different fluids. These compartments are surrounded by barriers made of epithelial cells, which are sealed together by tight junctions (TJs). On p. 5087, Shoichiro Tsukita and colleagues investigate how the establishment of the endolymph compartment of the mammalian cochlea is necessary for hearing. This compartment has a high K+ concentration and a positive endocochlear potential (EP). These characteristics, which are essential for transduction of acoustic signals to electrical signals by the cochlear hair cells, are thought to be generated by the stria vascularis, an adjacent compartment delineated by two epithelial cell layers. By making mice lacking claudin-11 - a major component of TJs in the basal cell layer of the stria vascularis - the authors show that, although an intact stria vascularis is not needed to maintain endolymph K+ concentrations, it is indispensable for the generation/maintenance of EP and thus for hearing.