When the kidneys can no longer remove waste, salt, and water appropriately, hemodialysis is required to complete this life-sustaining task. A reliable vascular access is necessary for successful hemodialysis treatment. The most reliable form of access is an arteriovenous fistula (AFV), but 60% of AVFs fail to mature for use. Mechanisms responsible for AVF failure include intimal hyperplasia and poor lumen expansion. Compromised vasoreactivity might play a role but this has never been evaluated. In our murine AVF model, the external jugular vein (EJV) is connected to the carotid artery (CA), establishing 3 component parts to the AVF, i.e., proximal CA, EJV, and distal CA. Upon AVF creation, the EJV is acutely exposed to higher arterial pressures and disturbed flow patterns, known causes of endothelial cell dysfunction. First we tested the hypothesis that endothelial function is impaired in the EJV from the AVF (i.e., EJV-AVF) vs. the naïve (EJV-Con) mouse. Three days after creating the AVF, using isometric tension procedures, dose-dependent vasocontraction to the TxA2 receptor agonist U46619 was robust in the EJV-Con (p<0.05) but was absent in the EJV-AVF segment. Because vasocontraction is requisite to subsequently test endothelium-dependent vasorelaxation, our hypothesis could not be evaluated. Our second hypothesis was that endothelial function is impaired in the distal vs. proximal CA, secondary to disturbed flow patterns. U46619 -evoked vasocontraction was greater (p<0.05) in the proximal vs. distal CA segment of the AVF. Acetylcholine-evoked vasorelaxation was greater (p<0.05) in distal vs. proximal CA, whereas responses to sodium nitroprusside were similar between groups. These findings do not support our hypothesis and suggest that endothelial function is better in the distal vs. proximal CA. Studies are ongoing to : (i) substantiate these findings 21 days after AVF creation; and (ii) define compensatory mechanisms of vasorelaxation in the distal CA. Supported by R01HL153244 NIH/NIDDK (TL, YTS, JDS).