In the present study, we analyzed the effect of conditioned media (CM) from bovine aortic endothelial cells exposed to laminar shear stress (SS) of 5 dyne/cm ² (SS5) or 15 dyne/cm ² (SS15) for 16 hours on smooth muscle cell (SMC) migration. In response to CM from bovine aortic endothelial cells exposed to SS5 (CMSS5) and SS15 (CMSS15), migration was 45±5.5 and 30±1.5 cells per field, respectively ( P <0.05). Similar results were obtained with SS of 2 versus 20 dyne/cm ² and also when SS of 5 and 15 dyne/cm ² lasted 24 hours. Platelet-derived growth factor (PDGF)-AA levels in CMSS5 and CMSS15 were 9±7 and 18±5 ng/10 ⁶ cells for 16 hours, respectively ( P <0.05); PDGF-BB levels in CMSS5 and CMSS15 were 38±10 and 53±10 ng/10 ⁶ cells for 16 hours, respectively ( P <0.05). PDGF receptor α (PDGFRα) and PDGF receptor β (PDGFRβ) in SMCs were phosphorylated by CMSS15>CMSS5. In response to CMSS15, a neutralizing antibody against PDGF-AA enhanced SMC migration to a level comparable to that of CMSS5; in contrast, antibodies against PDGF-BB abolished SMC migration. Transfection of SMCs with a dominant-negative PDGFRα or PDGFRβ increased or inhibited, respectively, SMC migration in response to CMSS15. Overexpression of wild-type PDGFRα inhibited SMC migration in response to CMSS5, CMSS15, or recombinant PDGF-BB ( P <0.001). These results suggest that the ability of high SS to inhibit arterial wall thickening in vivo may be related to enhanced activation of PDGFRα in SMCs by PDGF isoforms secreted by the endothelium.