The inner ear epithelium secretes potassium (K+) and absorbs sodium (Na+). The authors' experiments utilized, for the first time, isolation of the nonsensory from the sensory regions of the utricle in the vestibular labyrinth of the gerbil by means of injecting insulating seals. It was found that the isolated nonsensory region accumulated rubidium (Rb+) (as a marker for K+) in the endolymph to a level of eight times (41 mmol/l) that in the incubation medium (5 mmol/l) over a 90-minute period. However, Na+ rose during this period, suggesting that cells excluded from this preparation (sensory regions) may be involved in Na+ reabsorption. Further observations of the time course of changes in the luminal concentrations of cations suggested that the endpoints observed at 90 minutes represented a new steady state in which both Na+ and K+ concentrations were higher and lower, respectively, than under control conditions (by about 40 mmol/l each), but they were still each displaced from electrochemical equilibrium. This implies that the processes of Na+ absorption and/or K+ secretion may be shared in some way between the two regions. This finding is unexpected, based on previous models of endolymph production, in which ionic regulation has been assigned to nonsensory regions alone.