The viscoelastic properties of cultured endothelial cells exposed to shear stress were measured by the micropipette technique and analyzed using a standard linear viscoelastic model. Cells from porcine aorta were cultured on glass coverslips. A shear stress of 2 Pa was applied using a parallel-plate flow chamber. After flow exposure, the cells were detached from the coverslips and suspended in culture medium. The micropipette experiment was performed on single cells under an inverted microscope. The desired negative pressure was applied stepwise to the tip of the micropipette by opening a solenoid valve. The deformation process of cells in the micropipette was observed through a TV camera and recorded on a videotape. To obtain the viscoelastic parameters, a half-space model of an endothelial cell was used. The cell was assumed to be a homogeneous and incompressible material, and a standard linear viscoelastic model was employed to account for the viscoelastic response. Cells exposed to shear stress for 6 h became spherical in shape after detachment from the substrate. In the case of a 24 h exposure, about half of the detached cells retained an elongated shape upon detachment, with the others taking on a spherical shape. The elastic constants, as determined based on the model, were approximately two times higher for the elongated cells than for control cells from static culture, no-flow conditions, indicating that the elongated cells became stiffer. Enhanced viscous properties also were observed for the elongated cells. These viscoelastic properties are considered to be closely related to cytoskeletal structure. Spherical cells upon detachment, even those that had been exposed to shear stress for 24 h, did not show such significant changes in viscoelastic mechanical properties.