Amiloride-sensitive epithelial sodium channels (ENaC) are the major pathway for efflux of sodium from the alveolar lumen. The functional activity of these channels modulates the volume and composition of the alveolar subphase, and is essential to the perinatal clearance of fetal lung liquid. The channels are composed of three subunits (α, β, and γ), each encoded by a distinct mRNA transcript. Adminstration of dexamethasone prenatally has been shown to increase expression of the α subunit in fetal distal lung epithelium. We investigated the postnatal effects of dexamethasone administration on the expression of each of the ENaC subunits. Timed pregnant Sprague-Dawley rats were administered dexamethasone 4 mg/kg IP (or carrier) daily from gestational age 18 to 21. Rats were allowed to deliver and ENaC expression determined at various postnatal timepoints by Northern blot analysis of total lung RNA. Blots were hybridized with cRNA probes for each of the channel subunits and signal quantified by phosporimagery after normalization to GAPDH. Expression of all three ENaC subunits was increased relative to control the first postnatal day. By the third postnatal day no difference in expression between control and treated animals was detected. We subsequently investigated the effect of postnatal adminstration of dexamethasone to newborn rats on lung ENaC expression. Rat pups at various ages received dexamethasone 2 mg/kg IP (or carrier) and were sacrificed 24 h later. An age-dependent effect of dexamethasone on ENaC expression was observed. Maximal effect was observed in 1week old pups, with an 4.5 fold increase in the α and a 2.5 fold increase in the β and γ subunits relative to control. Little effect was observed in mature animals. In situ hybridization with a digoxigenin labeled probe for theα subunit localized expression to the conductive airway epithelium and type II pneumocytes. Cultured type II pneumocytes demonstrated a dose (0.001 to 0.1 μM) and time (0-24 h) dependent increase in expression of each subunit in response to dexamethasone in vitro. These data demonstrate that pulmonary expression of all three subunits of the ENaC is regulable by postnatal glucocorticoid administration.