Endothelium-dependent vascular reactivity has been recommended as a reliable indicator of vessel health. Ultrasound-based assessment of diameter response to shear stress measured as the proportion of brachial dilatation caused by reactive hyperemia (RH), is the gold standard for evaluating endothelial reactivity. However, the conventional approach does not account for the transient increase in blood flow and subsequent change in the transmural pressure that stimulates the endothelial cells to dilate the artery. In this work, simultaneous measures of blood flow and blood pressure during RH are used to evaluate the response of the brachial artery impedance. Endothelial reactivity is assessed using a three-element impedance blood flow model of the brachial artery. A significant change (> 30%) was observed in the impedance parameters in response to RH. The results were repeatable (ICC > 0.64) over 2 consecutive days for 10 participants. The results demonstrate the applicability of brachial impedance analysis in evaluating endothelial reactivity. This method also directly evaluates endothelial response to shear since the flow velocity and pressure increase precede the diameter change.