The volume response of vestibular dark cells of the gerbil to a hyposmotic challenge was investigated. Tissues including dark cells were perfused in preparations in which the perfusate had access to both sides of the epithelium and the height of the dark cell layer was measured as an indicator of its volume. We found that dark cells showed a fast and strong regulatory volume decrease (RVD) and prevented cell swelling in hypotonic media. This mechanism was dependent upon extracellular [K^-] and [Cl^-]. Ion selectivity of this mechanism was K⁺=Rb⁺>Cs⁺>Na⁺=NMDG⁺ (N-methyl-D-glucamine) for cations and Cl^-= SCN^-=NO₃^->>gluconate^- for anions. RVD of dark cells was inhibited by K⁺- channel blockers barium, quinidine and lidocaine, by Cl^--channel blockers 4-acetamido-4' -isothiocyanatostilbene-2, 2'-disulfonic acid and 4, 4'-diisothiocyanatostilbene-2, 2'-disulfonic acid, by an Na⁺-K⁺ ATPase inhibitor ouabain and by low temperature, but was not inhibited by a loop diuretic bumetanide, by carbonic anhydrase inhibitors acetazolamide and ethoxyzolamide, by a K⁺-channel blocker tetraethylammonium, by a Cl^--channel blocker 5-nitro-2 (3-phenylpropylamino) -benzoic acid or by an inhibitor of the Na⁺-H⁺ exchanger amiloride. These data suggest that the RVD of dark cells occurs via separate K⁺ and Cl^- channels which are different from those active under isosmotic condition, and is presumably activated by a hyposmotic stimulus.