Aquaporin-4 (AQP4) has been implicated in cytotoxic brain edema resulting from water intoxication, brain ischemia or meningitis. AQP4 inhibitors suitable for clinical use would thus be expected to help protect against brain edema. Here, we report the effect of inhibitors on water conduction by AQP4 and AQP1 reconstituted into liposomes. Acetazolamide (AZA), an inhibitor of sulfonamide carbonic anhydrase (CA), reversibly inhibits water permeation through AQP4, but not through AQP1. Methazolamide (MZA), another sulfonamide CA inhibitor similar in chemical structure to AZA, shows no significant effect on water conduction by AQP4 or AQP1. Our results thus demonstrate that AZA acts as a reversible inhibitor for AQP4-mediated water conduction and indicate that AZA is specific, at least to some degree, for AQP4. AZA may thus serve as a lead compound for the development of AQP4-specific inhibitors for clinical applications.