Little is known about endothelial responses to the impinging flow hemodynamics that occur at arterial bifurcation apices, where intracranial aneurysms usually form. Such hemodynamic environments are characterized by high wall shear stress (WSS >40 dynes/cm(2)) and high wall shear stress gradients (WSSG >300 dynes/cm(3)). In this study, confluent bovine aortic endothelial cells were exposed to impinging flow in a T-shaped chamber designed to mimic a bifurcation. After 24-72 h under flow, cells around the stagnation point maintained polygonal shapes but cell density was reduced, whereas cells in adjacent downstream regions exposed to very high WSS and WSSG were elongated, aligned parallel to flow, and at higher density. Such behavior was not blocked by inhibiting proliferation, indicating that cells migrated downstream from the stagnation point in response to impinging flow. Furthermore, although the area of highest cell density moved downstream and away from the impingement point over time, it never moved beyond the WSS maximum. The accumulation of cells upstream of maximal WSS and downstream of maximal WSSG suggests that positive WSSG is responsible for the observed migration. These results demonstrate a unique endothelial response to aneurysm-promoting flow environments at bifurcation apices.