The apparent tendency of intracranial aneurysms to form at apices of cerebral bifurcations and outer sides of curved vessels, has led to many speculations that high wall shear stress (WSS) damages the arterial wall, starting from the inner layer: the endothelial cells (ECs). However, little is known about EC response to high WSS and wall shear stress gradients (WSSG). In this study the influence of WSS and WSSG on endothelial cell biology was examined under the hypothesis that initiation of intracranial aneurysms involves a local functional alteration in endothelium mediated by fluid shear stress and shear stress gradients. For this purpose, a flow chamber with converging and diverging sections was designed to expose cultured bovine aortic endothelial cells to positive and negative WSSGs. The chamber was designed such that effects of gradients at constant magnitude could be studied and simultaneously compared with zero gradient regions. The flow experiments ran for 24 hours. Endothelial cell morphology was investigated in relation to local flow characteristics and shear stress forces. Individual cell morphology and cell shape distribution in the monolayers were assessed using a computerized image analysis system. When exposed to wall shear stress gradients, cultured endothelial cells exhibited a dramatic response to characteristics of the flow. It was found that cells respond differently to negative and positive gradients. While in positive WSSG regions ECs aligned away from the flow, they were very well aligned with the flow in negative WSSG regions of nearly same magnitude. ECs were also found aligned with the flow in low WSS regions but were almost aligned perpendicular to the flow in high WSS regions. This investigation is one of the first to qualitatively compare effects of positive and negative WSSGs on vascular endothelial cells.