Lung diffusing capacity for nitric oxide (DLNO) is used to measure alveolar membrane conductance (DMNO), but disagreement remains as to whether DMNO=DLNO, and whether blood conductance (thetaNO)=infinity. Our previous in vitro and in vivo studies suggested that thetaNO<infinity. We now show in a membrane oxygenator model perfused with whole blood that addition of a cell-free bovine hemoglobin (Hb) glutamer-200 solution increased diffusing capacity of the circuit (D) for NO (DNO) by 39%, D for carbon monoxide (DCO) by 24%, and the ratio of DNO to DCO by 12% (all P<0.001). In three anesthetized dogs, DLNO and DLCO were measured by a rebreathing technique before and after three successive equal volume-exchange transfusions with bovine Hb glutamer-200 (10 ml/kg each, total exchange 30 ml/kg). At baseline, DLNO/DLCO=4.5. After exchange transfusion, DLNO rose 57+/-16% (mean+/-SD, P=0.02) and DLNO/DLCO=7.1, whereas DLCO remained unchanged. Thus, in vitro and in vivo data directly demonstrate a finite thetaNO. We conclude that the erythrocyte and/or its immediate environment imposes considerable resistance to alveolar-capillary NO uptake. DLNO is sensitive to dynamic hematological factors and is not a pure index of conductance of the alveolar tissue membrane. With successive exchange transfusion, the estimated in vivo thetaNO [5.1 ml NO.(ml blood.min.Torr)(-1)] approached 4.5 ml NO.(ml blood.min.Torr)(-1), which was derived from in vitro measurements by Carlsen and Comroe (J Gen Physiol 42: 83-107, 1958). Therefore, we suggest use of thetaNO=4.5 ml NO.(min.Torr.ml blood)(-1) for calculation of DM(NO) and pulmonary capillary blood volume from DLNO and DLCO.