Severe-intensity exercise initiated from an elevated metabolic rate would be expected to enhance the proportional activation of higher-order (type II) muscle fibers. The purpose of this study was therefore to test the hypothesis that, compared to placebo (PL), NO3--rich beetroot juice (BR) supplementation would speed the phase II vo₂ kinetics (_p) and enhance exercise tolerance during severe-intensity exercise initiated from a baseline of moderate-intensity exercise. Nine healthy, physically-active subjects were assigned in a randomized, double-blind, crossover design to receive BR (140 mL/day, containing ~8 mmol of NO₃^-) and PL (140 mL/day, containing ~0.003 mmol of NO₃^-) for 6 days. On days 4, 5 and 6 of the supplementation periods, subjects completed a double-step exercise protocol that included transitions from unloaded-to-moderate intensity exercise (U->M) followed immediately by moderate-to-severe-intensity exercise (M->S). Compared to PL, BR elevated resting plasma nitrite concentration (PL: 65 ± 32 vs. BR: 348 ± 170 nM, P<0.01) and reduced the vo_{2 p} in M->S (PL: 46 ± 13 vs. BR: 36 ± 10 s, P<0.05) but not U->M (PL: 25 ± 4 vs. BR: 27 ± 6 s, P>0.05). During M->S exercise, the faster vo2 kinetics coincided with faster NIRS-derived muscle [deoxyhemoglobin] kinetics (; PL: 20 ± 9 vs. BR: 10 ± 3 s, P<0.05) and a 22% greater time-to-task failure (PL: 521 ± 158 vs. BR: 635 ± 258 s, P<0.05). Dietary supplementation with NO₃^--rich BR juice speeds vo₂ kinetics and enhances exercise tolerance during severe-intensity exercise when initiated from an elevated metabolic rate.