Serial arterial PO-2 values were obtained following discontinuation of supplemental inspiratory oxygen by face mask in 21 patients suffering from a variety of cardiac diseases. All patients were free of pulmonary disease. Arterial PO2 values returned to baseline within seven minutes after supplemental oxygen was withdrawn. Decay rate constants calculated from the arterial PO2 measurements were unaffected by cardiac index or the level of pulmonary capillary wedge pressure. We conclude that in patients free of overt pulmonary disease, arterial PO2 measured five to seven minutes after withdrawal of supplemental inspiratory oxygen accurately reflects an individual patient's baseline arterial PO2 value.