Heller, Hartmut, Gabi Fuchs, and Klaus-Dieter Schuster. Pulmonary diffusing capacities for oxygen-labeled CO₂ and nitric oxide in rabbits. J. Appl. Physiol. 84(2): 606-611, 1998.We determined the pulmonary diffusing capacity (DL) for ¹⁸O-labeled CO₂ (C¹⁸O₂) and nitric oxide (NO) to estimate the membrane component of the respective gas conductances. Six anesthetized paralyzed rabbits were ventilated by a computerized ventilatory servo system. Single-breath maneuvers were automatically performed by inflating the lungs with gas mixtures containing 0.9% C¹⁸O₂ or 0.05% NO in nitrogen, with breath-holding periods ranging from 0 to 1 s for C¹⁸O₂ and from 2 to 8 s for NO. The alveolar partial pressures of C¹⁸O₂ and NO were determined by using respiratory mass spectrometry. DL was calculated from gas exchange during inflation, breath hold, and deflation. We obtained values of 14.0 ± 1.1 and 2.2 ± 0.1 (mean value ± SD) ml · mmHg¹ · min¹ for DL_C¹⁸O2 and DL_NO, respectively. The measured DL_C¹⁸O2/DL_NO ratio was one-half that of the theoretically predicted value according to Graham's law (6.3 ± 0.5 vs. 12, respectively). Analyses of the several mechanisms influencing the determination of DL_C¹⁸O2 and DL_NO and their ratio are discussed. An underestimation of the membrane diffusing component for CO₂ is considered the likely reason for the low DL_C¹⁸O2/DL_NO ratio obtained.