(MIGET). The DLO 2 -to-DLCO ratio averaged 1.61. Correlation between DL O 2 (RB) and DLO 2 (MIGET) was similar in normal and post-resection groups. Overall, DL O 2 (MIGET) ϭ 0.975 DLO 2 (RB); mean difference between the two techniques was under 5% for both animal groups. We conclude that, despite various uncertainties inherent in these two disparate methods, the Roughton-Forster equation adequately predicts diffusive O 2 transfer from rest to heavy exercise in canines with normal, as well as reduced, lung capacities. oxygen-diffusing capacity; multiple inert-gas elimination technique; rebreathing technique; Roughton-Forster relationship; lung resection; dog THE IMPETUS FOR MEASURING lung diffusing capacity arose because two famous investigators (Christian Bohr and John Scott Haldane) reported in the 1890's using independent methods (2, 8) that O 2 was actively secreted into pulmonary capillary blood, so that O 2 tension of blood leaving the lung was higher than that in alveolar air. August and Marie Krogh (26,27) were skeptical of the secretion theory, owing to uncertainties in the techniques. Krogh and Bohr recognized a need to measure diffusing capacity using a gas other than O 2 to determine whether passive diffusion could support the levels of O 2 uptake during exercise. Haldane was the first to measure carbon monoxide (CO) uptake at rest on himself (7). Based on Haldane's measurements, Bohr (1) (24,30,42). Bohr also derived the famous Bohr integral to interpret the data but erroneously concluded that O 2 secretion was required to raise O 2 uptake above 1.5 l/min. Marie Krogh, using a breath-holding method, measured DL CO at rest and exercise (27) and showed that DL O 2 rose to two to four times that measured at rest by Bohr, sufficient to support O 2 uptake during exercise, even at high altitude, without invoking O 2 secretion. Thus diffusion was established as the process of O 2 transfer within pulmonary alveoli.Marie Krogh (27) in 1915 first proposed the use of DL CO to infer physiological limits of diffusive O 2 transfer, assuming DL O 2 Ϸ 1.23⅐DL CO , which took into account the relative tissue diffusivities for O 2 and CO, but not the relative conductance of capillary blood (⌰ O 2 and ⌰ CO , respectively, in ml gas⅐min Ϫ1 ⅐ml blood Ϫ1 ⅐mmHg Ϫ1 ). In the 1950's, Roughton and Forster (34) partitioned DL CO into separate resistances imposed by the alveolar membrane (Dm CO ) and capillary blood (⌰ CO ⅐Vc), i.e.,where ⌰ CO (ml CO⅐min Ϫ1 ⅐ml blood Ϫ1 ⅐mmHg Ϫ1 ) is the empirical CO uptake by whole blood measured in vitro, and Vc is pulmonary capillary blood volume (ml). Half a century later, the true values of ⌰ O 2 and ⌰ CO remain controversial, and no study has formally established whether Dm CO and Vc accurately predict diffusive O 2 transfer. Resistance of the blood for CO uptake is thought to be greater than that for O 2 uptake. There have been concerns that true Dm CO and Vc are sensitive to inspired O 2 tension (18), thereby introducing errors when inferring O 2 transfer from CO uptake measured at dif...