The p38/mitogen-activated protein (MAP) kinase-activated protein kinase 2 (MAPKAP kinase 2)/heat shock protein (HSP)25/27 pathway is thought to play a critical role in actin dynamics. In the present study, we examined whether p38 was involved in the morphological changes seen in endothelial cells (EC) exposed to shear stress. Cultured bovine aortic EC were subjected to 14 dyn/cm(2) laminar steady shear stress. Peak activation of p38, MAPKAP kinase 2, and HSP25 were sixfold at 5 min, sixfold at 5 min, and threefold at 30 min compared with static control, respectively. SB-203580 (1 microM), a specific inhibitor of p38, abolished the activation of MAPKAP kinase 2 and HSP25 as well as EC elongation and alignment in the direction of flow elicited by shear stress. The mean orientation angle of cells subjected to shear without SB-203580, with SB-203580, or static control were 17, 50, and 43 degrees, respectively (P < 0. 05). EC transfected with the dominant negative mutant of p38-alpha aligned randomly with no stress fiber formation despite exposure to shear stress. These data suggests that the pathway of p38/MAPKAP kinase 2/HSP25/27 is activated in response to shear stress, and this pathway plays an important role in morphological changes induced by shear stress.