Because immature, in contrast to mature, fetal lungs have ineffective Na transport, we wished to determine the ontogeny of Na(+)-K(+)-ATPase expression in fetal distal lung epithelium (FDLE). FDLE and fibroblasts (FLF) from 17- to 22-day gestational age fetal rats (term = 22 days) were grown in primary culture. Northern and slot-blot analyses utilizing isoform-specific cDNA probes determined that alpha 1- (3.7 kb) and beta 1- (2.7, 2.3, and 1.9 kb) transcripts were present in FDLE at levels approximately fivefold higher than in FLF. alpha 2-, alpha 3-, or beta 2-isoforms of Na(+)-K(+)-ATPase were not detected. In 17-day gestational age FDLE, only small amounts of alpha 1-mRNA levels were detectable, and there were approximately 10-fold less beta 1-isoform transcripts. By 20 days gestational age, the level of alpha 1-transcripts roughly doubled, whereas beta 1-levels increased approximately sixfold. Thus, during the transition from the canalicular to saccular stages of lung development, FDLE have a differentially regulated surge in mRNA levels of alpha 1- and beta 1-Na(+)-K(+)-ATPase isoforms and do not switch isoforms during lung development. Levels for both isoform transcripts then fell before birth, reaching values less than those seen for 17-day gestational age FDLE. FDLE vesicle Na(+)-K(+)-ATPase activity did not increase until 22 days gestational age.