Transforming growth factor beta (TGFbeta) contributes to tumor progression by inducing an epithelial to mesenchymal transdifferentiation (EMT) and cell migration. We found that TGFbeta-induced EMT was blocked by inhibiting activation of p38 mitogen-activated protein kinase (MAPK) with H-7, a protein kinase C inhibitor, and with SB202190, a direct inhibitor of p38MAPK. Inhibition of the p38MAPK pathway affected TGFbeta-mediated phosphorylation of ATF2, but did not inhibit phosphorylation of Smad2. SB202190 impaired TGFbeta-mediated changes in cell shape and reorganization of the actin cytoskeleton. Forced expression of dominant-negative (DN) MAPK kinase 3 (MKK3) inhibited TGFbeta-mediated activation of p38MAPK and EMT. Expression of DN-p38alpha impaired TGFbeta-induced EMT. Inhibition of p38MAPK blocked TGFbeta-induced migration of non-tumor and tumor mammary epithelial cells. TGFbeta induced activation of the p38MAPK pathway within 15 minutes. Expression of TGFbeta type II (TbetaRII) and type I (TbetaRI/Alk5) kinase-inactive receptors blocked EMT and activation of p38MAPK, whereas expression of constitutively active Alk5-T204D resulted in EMT and phosphorylation of MKK3/6 and p38MAPK. Finally, dominant-negative Rac1N17 blocked TGFbeta-induced activation of the p38MAPK pathway and EMT, suggesting that Rac1 mediates activation of the p38MAPK pathway. These studies suggest that the p38MAPK pathway is required for TGFbeta-mediated EMT and cell migration.