The endolymph differs from all other extracellular fluids in its positive potential and in the ionic compositions in the various parts of the endolymphatic space. The endolymphatic space is surrounded by a tight neuroepithelium. The strial cells, which have a negative intracellular potential of -67 mV, generate a positive endolymphatic potential of +80 mV. The potassium ions are pumped into the endolymph by an active energy-consuming process. Intracellular ion concentrations of the marginal strial cells are 135 mM for potassium and 6 mM for sodium, which are comparable to concentrations in other epithelial cells. During anoxia or after ethacrynic acid injection potassium concentrations in the cochlear endolymph and in the marginal strial cells decrease, while sodium concentrations increase.