Acetazolamide was given in the early stage of ischemic cerebral edema produced by unilateral permanent carotid occlusion in gerbils. The animals were studied 1, 4, and 6 hours after ischemia. The tissues were examined for water and electrolyte concentrations and ischemic areas were visualized by 2,3,5-triphenyltetrazolium chloride (TTC) and H-E staining. Acetazolamide injected just after occlusion showed a positive effect in reducing edematous changes. Later administration of the drug had neither positive nor harmful effect on the ischemic brains. Thus, acetazolamide seems to be useful for cerebrovascular response studies in the early stage of a brain lesion.