Carlen's tubes were modified by introducing a manometer line into each lumen to allow both bronchi to be insufflated continuously. During such continuousflow ventilation (CFV) with a gas flow of 1 L/kg body weight min in 6 anesthetized dogs, both CO2and O2homeostasis were maintained. By contrast, during apneic oxygenation the mean Paco2rose by approximately 6 torr/min. After 12 min of apneic oxygenation the introduction of CFV at 1 L/kg min reduced the mean Paco2from 120 to 38 torr within 6 to 9 min; by 15 to 30 min the Paco2was stable at 33 torr. The mean Pao2was stable at 80 torr and 451 torr using air and oxygen, respectively, as the insufflating gas. When the CFV was reduced to 0.75 L/kg ± min, there was a rise in mean Paco2from 34 to 53 torr in 12 min; Paco2was then reduced to 40 torr when 4 breath/min was introduced with a mean tidal volume of 375 ml.