The pH value and the DC potential in the endolymphatic sac (ES) were measured using H+-selective double-barreled electrodes and the intra- and extradural approach, the pH value in the ES fluid (6.65 ± 0.12, n = 7) was markedly lower than that in the cerebrospinal fluid (7.39 ± 0.07, n = 7). Acidity in the ES fluid, under normal (nontreated) conditions may be closely associated with metabolism in the epithelium of the ES.