After obliteration of endolymphatic sac and duct, no significant alteration in d.c. potential, K+ activity or protein content during the development of endolymphatic hydrops could be observed. The K activity of endolymphatic sac was only one-ninth of that in the cochlear part of the endolymph. After injection of Thoro-trast into the endolymphatic sac, aggregated particles were found in the cochlear endolymph 2 days later. Etha-erynic acid (60 mg/kg) caused a decrease in K activity in the cochlear endolymph, but an increase in the endolymphatic sac. Intercellular edema was observed both in stria vascularis and in endolymphatic sac by light and electronmicroscopy after ethacrynic acid administration. These results suggest the existence of an active transport mechanism in the endolymphatic sac epithelium.