At birth, fetal distal lung epithelial (FDLE) cells switch from active chloride secretion to active sodium (Na+) reabsorption. Sodium ions enter the FDLE and alveolar type II (ATII) cells mainly through apical nonselective cation and Na+selective channels, with conductances of 4-26 pS (picoSiemens) in FDLE and 20-25 pS in ATII cells. All these channels are inhibited by amiloride with a 50% inhibitory concentration of < 1 μM, and some are also inhibited by [N-ethyl-N-isopropyl]-2'-4'-amiloride (50% inhibitory concentration of <1 μM). Both FDLE and ATII cells contain the α-,α-, and γ-rENaC (rat epithelial Na+ channels) mRNAs; reconstitution of an ATII cell Na+-channel protein into lipid bilayers revealed the presence of 25-pS Na+ single channels, inhibited by amiloride and [N-ethyl-N-isopropyl]-2'-4'- amiloride. A variety of agents, including cAMP, and in some cases Ca2+, increased the activity and/or oxygen, glucocorticoids, rENaC mRNA levels. The phenotypic properties of these channels differ from those observed in other Na+ -absorbing epithelia. Pharmacological blockade of alveolar Na+ transport in vivo, as well as experiments with newborn α-rENaC knock-out mice, demonstrate the importance of active Na+ transport in the reabsorption of fluid from the fetal lung and in reabsorbing alveolar fluid in the injured adult lung. Indeed, in a number of inflammatory diseases, increased production of reactive oxygen-nitrogen intermediates, such as peroxynitrite (ONOO-), may damage ATII and FDLE Na+ channels, decrease Na+ reabsorption in vivo, and thus contribute to the formation of alveolar edema.