The effects of glycerol were investigated in guinea pigs in which the endolymphatic sac (ES) and duct (ED) had been obliterated unilaterally. Twenty-four hours after obliteration of the right ES and ED, glycerol was injected intraperitoneally. One to two hours after administration of glycerol, collapse or folding of Reissner’s membrane was observed in the operated cochlea. No remarkable change was found in the nonoperated ear. The lumen of the ES of the nonoper-ated ear collapsed with intraluminal stainable substance within 1 h. The volume of the stainable substance increased with time and could still be seen 4 h after the injection of glycerol. Twenty-four hours after the injection of glycerol, the configuration of the ES was normal again. In addition, the animals showed marked unilateral (operated side) vestibular dysfunction after injection of glycerol. These findings suggest that the ES also has a secretory capacity and plays an important role in inner ear fluid volume and pressure regulatio