The effects of hyperosmolar substances on the ultrastructure of the endolymphatic sac were studied in mice. Fifteen minutes after intravenous injection of urea or glycerol, subcellular changes in the endolymphatic sac were observed. These consisted of the occurrence of abundant cytoplasmic granules with a floccular or lamellar material, or both, in the light epithelial cells. Similar material was also present in the lumen of the endolymphatic sac, suggesting a common source and increased secretory activity. Mannitol caused similar changes, though less pronounced. The possibility that the alterations in the fine structure of the endolymphatic sac may be associated with a reduction in the hydrostatic fluid pressure in the rest of the labyrinth is discussed.