The transepithelial potential in the endolymphatic sac (ESP) was recorded up to 60 min after apical injection of ouabain, bumetanide, quinine, barium, tetraethylammonium, and 4-aminopyridine. After control injection, ESP decreased by 74% and completely recovered at 30 min. After ouabain, barium, or quinine injection, the ESP time course was similar to that in the control group. After bumetanide, tetraethylammonium, or 4-aminopyridine injection, complete recovery was only observed at 60 min. These results suggest that apical K+ conductance and Na-K-2Cl cotransporter could be involved in the genesis of ESP.