Endothelial dysfunction is common in congenital heart disease (CHD). Isometric handgrip (IHG) exercise may improve endothelial function; however, whether this applies to patients with CHD is unknown. We investigated brachial artery flow-mediated dilation (FMD) pre- and post- acute IHG exercise using duplex ultrasound in CHD and healthy controls (CTL; n=19 (7 F) vs. 21 (9 F), age: 13±4 vs. 13±4 y). Whereas IHG improved mean FMD in controls, it had no effect on the mean FMD response in patients with CHD. Subsequently, participants within each group were categorized based on ΔFMD response (post-pre IHG FMD), in which those that experienced a decrease were classified as non-responders (NRESP) and those that experienced an increase were classified as responders (RESP). Among NRESP, ΔFMD was lower in CHD vs. controls ( p=0.0224). Among RESP, ΔFMD was lower in CHD vs. controls ( p=0.0543). Data during pre-occlusion in NRESP from pre- to post-IHG exercise showed improved antegrade shear rate (condition effect, p=0.0078), retrograde shear rate (interaction effect, p=0.0410), and oscillatory shear index (interaction effect, p=0.049) in CHD compared to controls. Potentially, IHG exercise enhanced vascular sensitivity to shear, resulting in an increase in baseline diameter and reduced FMD in CHD NRESP. Thus, reduced FMD after acute IHG exercise is influenced by improvements in resting shear rate parameters in patients with CHD. Saskatchewan Health Research Foundation, Heart and Stroke Foundation of Canada. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.