1. Measurements of the CO(2) tensions in arterial blood, in blood from the superior sagittal sinus, in cisternal cerebrospinal fluid, and on the surface of the cerebral cortex were made in spontaneously breathing anaesthetized cats and rats after inhibition of carbonic anhydrase with acetazolamide in doses of 50-150 mg/kg.2. After an acetazolamide dose of 50 mg/kg in cats there was a mean increase in the c.s.f. CO(2) tension of 8 mm Hg; after a dose of 100 mg/kg the corresponding mean increase was 13 mm Hg.3. Since the CO(2) tension measured in venous blood was only moderately influenced by the acetazolamide, the normal P(CO2) difference between venous blood and c.s.f. was markedly reduced. The apparent arterial CO(2) tension, i.e. the CO(2) tension measured in vitro, always changed to the same extent as the c.s.f. CO(2) tension.4. The findings were confirmed by measurements of the CO(2) tension on the surface of the cerebral cortex, and by measurements of the blood and c.s.f. CO(2) tensions in the rat.5. It is concluded that the mean tissue CO(2) tension of the brain can be estimated from the measured arterial CO(2) tension, even under conditions of carbonic anhydrase inhibition.