To evaluate the retention of carbon dioxide in tissue during the reduction of carbonic anhydrase activity following the administration of 5, 10, 20, or 30 mg/kg of acetazolamide in dogs, we measured carbon dioxide pressure (PCO2) in arterial blood, mixed venous blood, alveoli, and tissue. Respiration was maintained at a constant level. In the control (noninjected) group, PCO2 in tissue did not change for 3 hours under controlled respiration. Following the injection of 5 to 30 mg/kg of acetazolamide, PCO2 increased in arterial blood, mixed venous blood, and tissue in a dose-related manner, and decreased in the alveoli. The (a-et)PCO2 widened to 21.9 +/- 1.0 mmHg from 0.6 +/- 1.0 mmHg, and the (t-v)PCO2 to 16.1 +/- 2.0 mmHg from 5.1 +/- 0.6 mmHg, in response to acetazolamide. It is suggested that carbon dioxide is retained in tissue when carbonic anhydrase activity is inhibited by acetazolamide.