Following inhibition of blood carbonic anhydrase by acetazolamide in anesthetized dogs with one lung rebreathing in connection with a bag and with the other lung ventilating freely on pure oxygen, mixed venous Pco2 increased and alveolar Pco2 decreased. Carbon dioxide output dropped initially but gradually rose and attained a steady state value equal to that of the initial control condition. Before acetazolamide, 19% of the pulmonary CO22 output was derived from dissolved CO2, 26% from carbamino compound, and 55% from bicarbonate. In the new steady state following acetazolamide, 45% came from dissolved CO2, 38% from carbamino, and 17% from bicarbonate. Submitted on June 12, 1961