In the absence of erythrocytes, carbonic anhydrase (CA) localized to the pulmonary capillary endothelium catalyzes the dehydration of bicarbonate to CO2. We studied the effects of lung CA and the reactions of CO2 on CO2 excretion in isolated lungs perfused with buffer. In indicator-dilution experiments, recoveries of dissolved CO2 and acetylene (C2H2) in the venous effluent were delayed significantly compared with a vascular indicator because the gases were distributed in both the vascular and alveolar volumes. In a second group of experiments, the kinetics of CO2 excretion were monitored with a plethysmographic method after injection of a bolus containing dissolved CO2 or bicarbonate. Exchange was compared with excretion of dissolved C2H2. The rate of excretion of dissolved CO2 and C2H2 was identical, indicating that CO2 is exchanged in the same manner as an inert gas. When bicarbonate was injected, CO2 excretion lagged behind C2H2 excretion by approximately 0.3 s. Inhibition of lung CA with acetazolamide reduced the quantity of CO2 exchanged to one-fourth of control and decreased the delay in exchange by one-half.