The signaling pathways by which mechanical forces modulate fetal lung development remain largely unknown. In the present study, we tested the hypothesis that strain-induced fetal type II cell differentiation is mediated via the cAMP signaling pathway. Freshly isolated E19 fetal type II epithelial cells were cultured on collagen-coated silastic membranes and exposed to mechanical strain for varying intervals, to simulate mechanical forces during lung development. Unstretched samples were used as controls. Mechanical strain activated heterotrimeric G-protein _s subunit, cAMP and the transcription factor CREB. Incubation of E19 cells with the PKA inhibitor H-89 significantly decreased strain-induced CREB phosphorylation. Moreover, adenylate cyclase 5 and CREB genes were also mechanically induced. In contrast, components of the PKA-independent (Epac) pathway, including Rap-1 or B-Raf, were not phosphorylated by strain. The addition of forskolin or dibutyryl cAMP to unstretched E19 monolayers markedly upregulated expression of the type II cell differentiation marker surfactant protein-C (SP-C), whereas the Epac agonist 8-pCPT-2-O-Me-cAMP had no effect. Furthermore, incubation of E19 cells with the PKA inhibitor Rp-8-MB-cAMPS or transient transfection with plasmid DNA containing a PKA inhibitor (PKI) expression vector significantly decreased strain-induced SP-C mRNA expression. In conclusion, these studies indicate that cAMP-PKA-dependent signaling pathway is activated by force in fetal type II cells and participates in strain-induced fetal type II cell differentiation.