A rapidly responding CO2 sensor and cuvette were placed in the carotid artery of anaesthetized cats and changes of PCO2 in post-pulmonary capillary blood were recorded when flow through the cuvette was suddenly stopped. 2. Under control conditions when the cats breathed spontaneously or were ventilated artificially, stop-flow caused Pa, CO2 to decay by 2--5 mmHg reaching a new equilibrium in 10--15 sec. The amount by which CO2 decayed was reduced by the inhalation of high O2. The decay was enhanced by hypoxia, the inhalation or infusion of CO2. It was reversed by the administration of acetazolamide: now with stop-flow, Pa, CO2 rose by 8--15 mmHg. 3. the mechanism of this decay is uncertain. We propose that it is due to the transfer of CO2 from plasma to red cells in post-capillary blood in response to the reduction of [H+]i as H+ binds to Hb following the oxylabile release of CO2 from Hb.
