Endothelial dysfunction, manifesting as attenuated flow mediated dilation (FMD), following ischemia-reperfusion (IR) injury is a clinically important measure of vascular health. Antioxidants may prevent this dysfunction by scavenging reactive oxygen species, however the acute effects of oral antioxidant administration in humans are unknown. Low-flow mediated constriction (L-FMC), a further parameter of endothelial health, is largely unstudied and the mechanisms for this response are unclear. This study assessed interventions that impact FMD and L-FMC throughout an IR injury protocol. Twelve healthy subjects (5 female, 7 male) attended the lab on 3 occasions. The initial test acted as a baseline measure. Prior to subsequent visits subjects ingested either: 1) an antioxidant cocktail (300mg α-lipoic acid, 500mg Vitamin C and 200IU Vitamin E) 2 hours prior and a further cocktail (300mg α-lipoic acid, 500mg Vitamin C and 400IU vitamin E) 1.5 hours prior; or 2) 1200mg of a prostaglandin inhibitor (Ibuprofen) 1 hour prior in a blinded fashion. Endothelial function was assessed by non-invasive ultrasound of the right brachial artery. For each vascular function protocol vessel diameters were determined throughout 5 min of forearm ischemia and 3 min after. Following a baseline measure (Pre) IR injury was induced by a 20 min upper arm occlusion. Subsequent vascular function protocols were carried out at 15, 30 and 45 min of recovery. Values are means ± SD, analysed by ANOVA. Endothelial dysfunction was evident in all three conditions. FMD was attenuated at 15 min post-IR injury (Control Pre: 5.96±3.41%; Post15: 0.67±3.26%; Antioxidant Pre: 6.90±3.83%; Post15: 0.68±2.87%; Ibuprofen Pre: 5.85±4.81%; Post15: -0.63±4.81%; p<0.05) but recovered by 45 min to near basal values. Antioxidant administration did not preserve FMD compared to control (p>0.05). The magnitude of L-FMC was augmented at 15 min compared to basal values (Pre: 1.44±0.27%; Post15: 3.75±1.73%; p<0.05) and recovered by 45 min. Ibuprofen administration produced the largest constrictive response and a significant time x condition interaction was observed (Ibuprofen Pre: -1.13 ±1.71%; Post15: -5.57 ±3.82%; Control Pre: -1.58 ±2.26%; Post15: -3.58 ±2.65%; interaction: p<0.05). Results demonstrate the presence of endothelial dysfunction following IR injury and the inability of acute oral antioxidant supplementation to preserve vascular function. Data also suggests that a lack of shear stress during occlusion combined with prostaglandin blockade magnifies L-FMC; possibly due to augmented expression of endothelin-1. Moreover, when prostaglandins are blocked their habitual role in inhibiting endothelin-1 secretion is abolished, increasing endothelin-1 bioavailability, and leading to a more pronounced L-FMC.