Hyaluronan (HA) 0.5, 1, 1.9 and 4% was instilled into the round window niche in mice. Six hours after the 1.9% HA and 4, 6 and 8 h after the 4% HA deposition there was a significant increase in the proportion of granule-containing light epithelial cells in the endolymphatic sac. This indicated an increase in secretory activity. This activity could be a response to lowered intralabyrinthine pressure due to the osmotic capacity of HA. Secretion of high molecular substances can serve to keep the sac lumen patent and/or to regulate labyrinthine volume and pressure.