Endothelial dysfunction is an initial step of atherosclerosis and is associated with cardiovascular diseases. Measurement of flow-mediated vasodilation in the brachial artery using ultrasound is noninvasive and an accurate indicator of nitric oxide production. However, ultrasound based measurement sometimes depends on the observer’s skill. The purpose of this study was to evaluate response in reactive hyperemia employing viscoelastic indices, including stiffness and viscosity measured by strain-gauge plethysmography (SPG) in beat-to-beat. We measured viscoelastic parameters and pulse wave velocity (PWV) in 4 young (23±1 years) and 3 elderly (55±4 years) subjects. Our results showed that there were significant differences in viscosity after cuff deflation (0 to 100 sec) between the young and the elderly (p<0.001). However, there was no significant difference in PWV after cuff deflation between the young and the elderly (P=0.11). These findings suggest that the proposed viscoelastic indices represent the changes of arterial mechanical properties, which might be derived from flow-mediated vasodilation.