It is well known that the function of the cochlear and vestibular labyrinth depends on the high concentration of potassium (K+) in the luminal fluid, endolymph. Homeostasis of endolymphatic ion composition has been attributed to the stria vascularis and vestibular dark cells but with little prior experimental basis. The extremely small domain of each epithelial cell type bounding the endolymphatic space has precluded study of ion fluxes from these cells. The voltage-sensitive and (+)-selective vibrating probes were adapted recently for the demonstration of electrogenic K+ secretion and its regulation by stria vascularis and vestibular dark cell epithelium. The isolated stria vascularis and vestibular dark cell epithelium are known to produce a transepithelial current directed toward the endolymphatic side and this current has been shown to be sensitive to bumetanide, an inhibitor of the Na(+)-Cl(-)-K+ cotransporter. The vibrating probes were used to demonstrate that this current is carried by K+ and that the K+ flux is also sensitive to bumetanide. Several other agents and maneuvers which alter the transepithelial current (e.g. apical DIDS and basolateral hypotonic challenge) were found to produce similar changes in the K+ flux. The technique holds the promise of discovery of the contribution to the homeostasis of endolymph of other cell types in the inner ear.