The concept of apneic threshold implies that there is a threshold for arterial CO2 tension below which spontaneous breathing ceases. This paper describes a new method for measuring this threshold under clinical conditions in the operating room. Patients were ventilated so that arterial CO2 tension was maintained well below the apneic threshold. The ventilator was then turned off so that the patient's arterial CO2 rose during an apneic (no breathing) period according to his resting CO2 production. Prior to this apneic period, the patient breathed a high oxygen concentration, and during this period the patient's oxygenation was maintained by oxygen diffusion from the anesthetic circuit. The apneic threshold was determined as that value of arterial CO2 tension where spontaneous breathing commenced. The method has been tested in 15 patients and found to be useful for ventilating patients without muscle relaxants by keeping their arterial CO2 tension below the apneic threshold.