Fluid secretion from the pulmonary epithelium may play a significant role in determining intrauterine lung development. We used suspensions of distal pulmonary epithelial cells isolated from rat fetuses to assess a shift in secretory mechanisms occurring in the lung of this species during late gestation. The impact of cAMP on distal airway epithelial cells isolated from d 18 to d 21 rat fetuses was evaluated with measurements of cell volume and36Cl efflux rates. At d 18, 8-Br-cAMP stimulated a volume reduction measured by electronic cell sizing that was prevented by the Cl−channel blocker anthracene-9-carboxylate (A-9C) and reflected in an increased rate of A-9C sensitive36Cl efflux. Because the cystic fibrosis trans-membrane conductance regulator (CFTR) is thought to be a cAMP-regulated Cl−channel, we measured the effect of prior cell incubation with oligodeoxynucleotides antisense to the transcription site of the human CFTR gene on these events. We found that in antisense oligomer-treated cells, but not in sense oligomer-treated controls, volume and36Cl efflux responses to 8-Br-cAMP were prevented in d 18 cells. In d 21 cells, 8-Br-cAMP did not stimulate volume reduction but the calcium ionophore A23187 did elicit cell volume reduction in cells suspended in an isotonic Ca2+-containing medium that was prevented by A-9C. This response to the ionophore was not found in the d 18 cells, and incubation with the antisense CFTR oligomer had no effect on the ionophore-induced responses in d 21 cells. Our results suggest that CFTR mediates cAMP-stimulated Cl−conductance in d 18 rat fetal distal pulmonary epithelial cells. Furthermore, during fetal growth between d 18 and 21 there is a reciprocal pattern whereby Ca2+-activated Cl−conductance appears whereas CFTR disappears in the distal pulmonary epithelium.