To determine the respiratory effects of inhibiting carbonic anhydrase in brain tissue, we infused acetazolamide (ACZ) into the cisterna magna in anesthetized rats. Three concentrations, 4.5, 45 and 450 microM in artificial cerebrospinal fluid, were infused continuously at 2 microliter/min for 20 min and the responses to CO2 rebreathing trials were compared to sham CSF infusions without ACZ. Baseline, steady-state respiration during room air breathing was not affected by these doses of ACZ, but the two higher doses increased the minute ventilation rebreathing response. We conclude that inhibition of carbonic anhydrase in brain tissue increases the respiratory response to rebreathing CO2.