Lung growth and remodeling are modulated by mechanical stress, with fibroblasts thought to play a leading role. Little mechanistic information, however, is available about how lung fibroblasts respond to mechanical stress. Here we exposed cultured lung fibroblasts to tonic stretch and measured changes in phosphorylation status of mitogen-activated protein kinases (MAPKs), selected receptor tyrosine kinases (RTKs) and phospholipase C1 (PLC1), and activation of the small G-protein Ras. Human CCL-151 lung fibroblasts (LFs) were seeded on matrix-coated silicone membranes and exposed to equibiaxial 10-40% static stretch or 20% contraction in a custom-built device. Alternatively, LFs were stimulated with EGF, FGF2 or PDGF-BB, or exposed to stretch in the presence of inhibitors of EGFR (AG1478), FGFR (PD173074) and PDGFR (AG1296). Normalized p-ERK1/2, p-JNK, and p-p38 levels all increased following stretch but not contraction. Ligands to RTKs also broadly stimulated MAPKs, with the responses to EGF and PDGF most similar to stretch in terms of magnitude and rank order of MAPK responses. Stretching cells, however, failed to elicit measurable activation of EGFR, FGFR (FRS2 phosphorylation) or PDGFR. Moreover, potent inhibitors of the kinase activity of each receptor failed to attenuate stretch-induced MAPK activation. In addition, PLC1 and Ras, prominent effectors downstream of RTKs, were not activated by stretch. Our findings demonstrate that MAPKs are potently activated by stretch in lung fibroblasts, but in contrast to stress responses observed in other cell types, RTKs are not necessary for stretch-induced MAPK activation in LFs.