The adequacy of constant airway gas flow sustenance of arterial blood gas tensions was investigated in anesthetized-paralyzed mongrel dogs. Gas delivery was achieved via a main-stem bronchi cannulation system constructed of two polyethylene tubes bifurcating at the carina, which rested on the posterior surface of the trachea outside of an endotracheal tube positioned in the upper third of the trachea. Equal flows (total flow = Vin) of humidified air were delivered through each limb of the cannulation system at constant flow rates with Vin ranging from 8 to 28 l/min. Intratracheal pressures at these flows characteristically ranged from 0.1 to 1 cmH2O. Arterial O2 tension varied directly (PaO2 = 0.72 Vin + 74.6), and arterial CO2 tension varied inversely (PaCO2 = -0.73 Vin + 51.2) with Vin during ambient gas, constant-flow ventilation (CFV). During prolonged CFV (greater than 2 h), no evidence of CO2 accumulation or deterioration of PaO2, was observed. This study demonstrates that in apneic dogs normal blood gases can be achieved and maintained over prolonged periods with constant airway flow at low intratracheal pressures.