The contribution of reaction resistance to overall resistance to pulmonary carbon monoxide (CO) uptake [DL_CO/(_CO · Vc), where DL_CO is lung CO diffusing capacity, _CO is CO uptake conductance of erythrocytes, and Vc is pulmonary capillary blood volume] was determined in 10 anesthetized, paralyzed, and artificially ventilated rabbits. On the basis of the classical double-reciprocal equation of F. G. W. Roughton and R. E. Forster (J. Appl. Physiol. 11: 290-302, 1957), DL_CO/(_CO · Vc) was obtained by solving the relation DL_CO/(_CO · Vc) = 1  2/(DL_NO/DL_CO), where DL_NO/DL_CO represents the ratio between the respective single-breath diffusing capacities (DL) of nitric oxide (NO) and CO pulmonary capillary blood. The lungs of eight rabbits were inflated, starting from residual volume, by using 55 ml of indicator gas mixture (0.2% CO and 0.05% NO in nitrogen). DL values were calculated by taking the end-tidal partial pressures of CO and NO as analyzed by using a respiratory mass spectrometer. The overall value was DL_CO/(_CO · Vc) = 0.4 ± 0.025 (mean ± SD). Because of the use of O₂-free indicator gas mixtures, the end-tidal O₂ partial pressures were ~21 Torr. In one other rabbit, the application of 0.2% CO and 0.001% NO yielded DL_CO/(_CO · Vc) = 0.39; in the tenth rabbit, however, inspiratory volume was varied, and an identical value was found at functional residual capacity. We conclude that the contribution of reaction resistance to overall resistance to pulmonary CO uptake is independent of the inspiratory NO concentration used, including, with respect to the pertinent literature, the conclusion that in rabbits, dogs, and humans this contribution amounts to 40% when determined at functional residual capacity.