Homeostasis of endolymph volume is a complex mechanism, in which the endolymphatic sac (ES) may play an important role.To elucidate the effect of acute endolymphatic hydrops (EH) on the ES and to gain further information about the volume and pressure regulative function of the ES.Distilled water was injected into the middle ear cavity of adult CBA/J mice. The ESs were studied morphologically by light and transmission electron microscopy.Mild EH was found, particularly in the upper turn of the cochlea. Acute EH led to an increase in the size of the ES lumen, accompanied by collapse of the lateral intercellular spaces and dense perisaccular tissue, changes which had reversed 2 h after the injection.