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The newborn lung is cleared of fetal liquid by active Na+ transport. The heterotrimeric (α, β, γ) epithelial Na+ channel, ENaC, mediates this process. To understand the role of individual ENaC subunits in Na+ transport during development, we quantified murine ENaC (mENaC) subunit messenger RNA (mRNA) expression levels of fetal, neonatal, and adult mouse lung by Northern blot analysis and studied regional expression by in situ hybridization. αmENaC and γmENaC mRNA expression increased sharply in late fetal gestation and reached near-adult levels by Day 1 of postnatal life. βmENaC expression increased more gradually through late fetal and early postnatal life and increased progressively until adulthood. In situ hybridization studies showed similar localization patterns of αmENaC and γmENaC subunit expression in fetal and postnatal lung. γmENaC and αmENaC subunits were initially localized to fetal lung bud tubules and by late gestation both subunits were expressed in all regions (acinar and bronchiolar) of the distal lung epithelium. βmENaC was detected from 16 d gestation onward and was expressed most intensely in small airways. There was little expression of βmENaC in the alveolar region. In postnatal lung all three subunits were expressed intensely in small airways. In adult lung, αmENaC and γmENaC were expressed in a pattern consistent with an alveolar type II (ATII) cell distribution. The timing of quantitative changes in mENaC subunit expression is consistent with a role of Na+ transport in liquid clearance of the perinatal lung. Intense expression of mENaC subunits in medium and small airway epithelium and in ATII cells suggests that these regions are a primary location for liquid absorption in the perinatal and postnatal murine lung.