Regulation of active Na+ transport across fetal distal lung epithelial cells (FDLE) by corticosterone (CST), corticotropin-releasing hormone (CRH), and oxygen tension may be crucial for postnatal adaptation. FDLE isolated from 19-day rat fetuses (term: 22 days) were grown on permeable supports to confluent monolayers (duration 3 days) in 2.5, 5, 12, or 20% O2 with 5% CO2-balance N2 and mounted in Ussing chambers for measurement of short-circuit currents (Isc). FDLE monolayers grown in 20% O2 had significantly higher levels of total Isc and of their amiloride-sensitive (Iamil) and ouabain-sensitive (Iouab) components than hypoxic cells. Values (μA/cm2 ± SE) for 2.5-5% O2 and 20% O2 were, respectively, Isc 5.3 ± 0.2 vs. 8.4 ± 0.3 (P < 0.001), Iamil 3.4 ± 0.2 vs. 4.3 ± 0.2 (P < 0.01), and Iouab 3.4 ± 0.6 vs. 9.1 ± 0.6 (P < 0.001). Addition of CST but not CRH to the culture medium at any O2 concentration increased Iamil. FDLE cells grown at 5% O2 expressed significantly lower levels of α-, β-, and γ-epithelial Na+ channel (ENaC), and of the α1-Na+-K+-ATPase, as determined by Western blotting. We conclude that higher O2 concentrations increased total vectorial Na+ transport, and the function of Na+-K+-ATPase and apical amiloride-sensitive Na+ conductance, whereas CST only increased ENaC function.