Introduction: Dietary nitrate (NO3-) supplementation serves as an exogenous source of nitrite (NO3-) and nitric oxide (NO) through the NO3- NO3- NO pathway, and may improve vascular functions during normoxia. The effects of NO3- supplementation in healthy lowlanders during hypobaric hypoxia are unknown. Purpose: Determine the effect of acute oral NO3- supplementation via beetroot juice (BJ) on endothelial function (flow mediated dilation; FMD) in lowlanders at 3700 m. Methods: FMD was measured using ultrasound and Doppler in the brachial artery of 11 healthy subjects (4 females, age 25 +/- 5 yrs; height 1.8 +/- 0.1 m, weight 72 +/- 10 kg) sojourning to high altitude. In a randomized, double-blinded crossover study design, FMD was measured 3 h after drinking BJ (5.0 mmol NO3-) and placebo (PL; 0.003 mmol No-3(-)) supplementation at 3700 m, with a 24-h wash out period between tests. FMD was also measured without any BJ supplementation pre-trek at 1370 m, after 5 days at 4200 m and upon return to 1370 m after 4 weeks of altitude exposure (above 2500 m). The altitude exposure was interrupted by a decent to lower altitude where subjects spent two nights at 1370 m before returning to altitude again. Results: Ten subjects completed the NO3- supplementation. FMD (mean +/- SD) pre-trek value was 6.53 +/- 2.32% at 1370 m. At 3700 m FMD was reduced to 3.84 +/- 1.31% (p < 0.01) after PL supplementation but was normalized after receiving BJ (5.77 +/- 1.14% (p = 1.00). Eight of the subjects completed the interrupted 4-week altitude stay, and their FMD was lower at 4200 m (FMD 3.04 +/- 2.22%) and at post-altitude exposure to 1370 m (FMD 3.91 +/- 2.58%) compared to pre-trek FMD at 1370 m. Conclusion: Acute dietary NO3- supplementation may abolish altitude-induced reduction in endothelial function, and can serve as a dietary strategy to ensure peripheral vascular function in lowland subjects entering high altitude environments. (C) 2015 Elsevier Inc. All rights reserved.