F etuses of streptozotocin-induced diabetic rats exhibited delayed lung maturation and a 40% reduction in the steady-state level of lung Na+,K(+)-ATPase alpha 1 subunit mRNA and Na+,K(+)-ATPase activity at 21 d of gestation. In in situ hybridization experiments the signal specific for Na(+)-pump alpha 1 subunit message was strongest above columnar epithelial cells of air-conducting structures. Strong labeling was also present above cuboidal cells lining the forming alveoli, but not above mesenchymal cells. Immunocytochemical localization of the protein paralleled the distribution of the mRNA. Mesenchymal cells were more abundant in fetal lungs of diabetic mothers, and thus the decreased overall levels of Na+,K(+)-ATPase may result from the observed morphological pulmonary immaturity. One day after birth there was no apparent difference in lung morphology at the light microscopic level, in the localization or the steady-state level of Na+,K(+)-ATPase alpha 1 isoform mRNA, or in enzyme activity. Na+,K(+)-ATPase has a likely role in the active phase of fluid absorption in the airways of newborns before the onset of breathing. Decreased fluid clearance and lack of thinning of the lung's connective tissue may contribute to the increased risk for respiratory distress in infants of diabetic mothers.