The effect of acetazolamide on the ultrastructures of the murine endolymphatic sac was investigated. The animals were given a single intravenous dose of acetazolamide (100 mg/b.w.) and were sacrificed 0, 15, 30, 60 and 120 min after the injection, respectively. Prominent changes in the fine structure of the epithelial cells could be observed after 30 min. These alterations were even more pronounced after 1 h. After 2 h, the normal cell structure became to be reestablished. The most conspicuous change was a general reduction in the electron density of the dark cells. This was accompanied with a decreased number of cell organelles, especially ribosomes. Some light cells also underwent temporal modifications in their structure in the form of a reduced nuclear stainability associated with a loss of pinocytotic vesicles near the apical plasmalemma. In general, the dark cells seemed to be more influenced by acetazolamide than the light cells. The possibility that the dark cell changes are related to a modification of transepithelial ion and water flow is discussed.