To learn modes of CO2elimination during cardiac arrest, we continuously measured end-tidal CO2concentration (ETco2) in acutely arrested dogs with constant ventilation. A decrease in peak ETco2during cardiac arrest in each dog showed a washout biexponential function when graphed on semilog paper. The average half-times of each compartment were 19.2 ± 3.1 (sd) sec for the fast compartment and 108.1 ± 23.8 sec for the slow compartment; the fast compartment of the CO2elimination curve suggested that CO2was eliminated from the functional residual capacity, while the slow compartment Indicated CO2elimination from the pulmonary capillary blood and tissue stores. Neither pretreatment with sodium bicarbonate (1 mEq/kg iv) nor a 5-min cardiorespiratory arrest altered the mode of CO2elimination. The ETco2also reflected the potential effects of external cardiac compressions on pulmonary blood flow, as previously reported. Besides mixed venous blood CO2flowing back to the lungs by cardiac compressions, it should be noted that both alveoli and pulmonary capillary blood CO2are also reflected in the ETco2during the first minute of CPR.