We compared the volume of the pulmonary extravascular, extracellular water space using sodium and sucrose indicators in 8 normal and 11 edematous rabbit lungs by steady-state techniques. The volume of the extravascular, extracellular space (EVECW), expressed in milliliters per gram dry lung, was determined from the volume of distribution of 24Na, [3H]sucrose, or total sodium in 1) supernatant solutions obtained by centrifugation of homogenized lung or 2) intact lung fragments. There was no significant difference in EVECW measured in normal lungs [24Na 1.88 +/- (SD) 0.31; [3H]sucrose 1.84 +/- (SD) 0.27] or edematous lungs [24Na 4.78 +/- (SD) 1.44; [3H]sucrose 4.41 +/- (SD) 1.33] when comparing supernate solutions of homogenized lungs. Similarly, there was no significant difference between the volumes of distribution (EVECW) of these two isotopes when measured in intact fragments of normal lung [24Na 2.08 +/- (SD) 0.28; [3H]sucrose 1.68 +/- (SD) 0.37]. The volume of EVECW measured in supernates was less than in fragments with both isotopes. Washing the centrifuged tissue pellet and adding the wash activity to the initial supernate activity yielded results that were equal to the fragment technique. The failure to detect significant differences between 24Na and [3H]sucrose spaces in the lung suggests that the contribution on intracellular sodium to the measurement of lung water is small and relatively insignificant, at least within the confines of this experiment.