The hypothesis that fluid reabsorption from the air spaces is mediated at least in part by active transport of Na+ was investigated in six sets of experiments conducted in isolated fluid-filled rat lungs. Fluid reabsorption was monitored by following the changes in the air space concentration of labeled albumin. We found that incorporation of bicarbonate rather than a nonvolatile buffer (N-2-hydroxy-ethylpiperazine-N'-2-ethanesulfonic acid) in the air space solution more than doubled the rate of fluid reabsorption. Addition of 10(-4) M amiloride to the air space solution reduced the rate of fluid reabsorption over a 2-h experiment from 1.2 +/- 0.1 to 0.7 +/- 0.1 ml and decreased reabsorption of both labeled and unlabeled Na+ from the air spaces. To show that Na+ could be reabsorbed from the air spaces even if the concentrations of Na+ in the perfusate increased above those in the air space, mannitol (150 mM) was added to the perfusate and air space solutions and the concentrations of Na+ and Cl- were reduced to 90 and 60 mM, respectively. Mannitol diffuses across the pulmonary epithelium very slowly, and it osmotically restrained the movement of water out of the air spaces. Na+ concentrations in the perfusate increased by 10 +/- 2 mM, but concentrations in the air space remained unchanged. Despite an increasingly unfavorable concentration gradient for Na+, 0.2 mmol Na+ and 0.6 ml water were reabsorbed from the air spaces in 2 h. Ouabain (10(-4) M) did not appear to slow fluid reabsorption in the presence of mannitol, but it reduced K+ secretion into the air spaces and increased K+ appearance in the perfusate in a manner consistent with inhibition of Na+-K+-adenosinetriphosphatase at the basolateral surface of the epithelial cells. Fluid reabsorption was not altered when the lungs were exposed to a hypotonic solution (185 mM), but secretion of K+ into the air spaces was accelerated and K+ was lost from the perfusate. These experiments are consistent with active Na+ transport from the air spaces.