Dietary nitrate (NO3 -) supplementation via beetroot juice (BR) has been reported to lower oxygen cost (i.e., increased exercise efficiency) and speed up oxygen uptake (VO2) kinetics in untrained and moderately trained individuals, particularly during conditions of low oxygen availability (i.e., hypoxia). However, the effects of multiple-day, high dose (12.4 mmol NO- 3 per day) BR supplementation on exercise efficiency and VO2 kinetics during normoxia and hypoxia in well-trained individuals are not resolved. In a double-blinded, randomized crossover study, 12 3 well-trained cyclists (66.4 ± 5.3 ml min-1∙kg-1) completed three transitions from 6 rest to moderate-intensity (~70% of gas exchange threshold) cycling in hypoxia 38 and normoxia with supplementation of BR or nitrate-depleted BR as placebo. 3 Continuous measures of VO2 and muscle (vastus lateralis) deoxygenation (ΔHHb, 9 using near-infrared spectroscopy) were acquired during all transitions. Kinetics of 4 VO2 and deoxygenation (ΔHHb) were modelled using mono-exponential 1 functions. Our results showed that BR supplementation did not alter the primary time constant for VO2 or ΔHHb during the transition from rest to moderate- intensity cycling. While BR supplementation lowered the amplitude of the VO2 response (2.1%, p=0.038), BR did not alter steady state VO2 derived from the fit (p=0.258), raw VO2 data (p=0.231), moderate intensity exercise efficiency 47 (p=0.333) nor steady state ΔHHb (p=0.224). Altogether, these results demonstrate that multiple-day, high-dose BR supplementation does not alter exercise Journal Pre-proof efficiency or oxygen uptake kinetics during normoxia and hypoxia in well-trained athletes.