Carbon monoxide combines with oxyhemoglobin at a measurable rather than an infinitely fast rate (1, 2). This fact is important when carbon monoxide is used to estimate pulmonary diffusing capacity. If the rate at which this gas crosses the membrane between alveoli and pulmonary capillaries is greater than the rate at which it can react with oxyhemoglobin, carbon monoxide tension will rise in these capillaries and slow further diffusion. Therefore, the diffusing capacity of the lungs for carbon monoxide is dependent not only on the thickness and area of the membrane between alveoli and capillaries, but also on the volume of blood in the capillaries and the rate at which this blood can react with carbon monoxide. Roughton and Forster (3) have recently shown that pulmonary capillary blood volume and the true diffusing capacity of the pulmonary membrane can be computed from data on the apparent diffusing capacity of the lung for carbon monoxide (DLco) at different oxygen tensions and in vitro values for the rate of reaction of carbon monoxide with oxyhemoglobin contained in red cells (6) Such

The pulmonary diffusing capacity for oxygen (DLo,) is of great physiological and clinical significance (1). Its measurement, however, is relatively complex (2). Pulmonary diffusing capacity for carbon monoxide (DLco) which, it is usually assumed,' can be converted to DL02 from the known solubilities and molecular weights of the two gases, is, in general, more readily measured. Further, since the rate of reaction of carbon monoxide with hemoglobin has been measured (4) in vitro while that of oxygen has not, determination of DLco at two or more alveolar oxygen tensions makes possible the computation of pulmonary capillary blood volume and pulmonary membrane diffusing capacity (5-7).The methods used for measuring DLco are the single breath technique of Krogh (8) as modified by Ogilvie, Forster, Blakemore and Morton (9), the steady state technique using an independent measurement of the physiological dead space (10), the steady state technique using an alveolar sample (11) and a rebreathing technique using C'40 (12). The present paper presents a modification of the rebreathing method utilizing stable CO and continuous analysis.