AbstractTo investigate the effect of acute capsaicin (CAP) supplementation on time to exhaustion, physiological responses and energy systems contribution during continuous high-intensity exercise session in runners. Fifteen recreationally-trained runners completed two randomized, double-blind continuous high-intensity exercises at the speed eliciting 90% V̇O2peak (90% s V̇O2peak), 45 minutes after consuming capsaicin or an isocaloric placebo. Time to exhaustion, blood lactate concentration, oxygen consumption during and 20-min post-exercise, energy systems contribution, time to reach V̇O2peak, heart rate and the rate of perceived exertion (RPE) were evaluated. There was no significant difference between conditions for time to reach V̇O2peak (CAP:391.71±221.8 vs. PLA:298.20±174.5 sec, ES:0.58, p=0.872), peak lactate (CAP:7.98±2.11 vs. PLA:8.58±2.15 µmol, ES:−0.28, p=0.257), time to exhaustion (CAP:654.28±195.44 vs. PLA:709.20±208.44 sec, ES:−0.28, p=0.462, end-of-exercise heart rate (CAP:177.6±14.9 vs. PLA:177.5±17.9 bpm, ES:−0.10, p=0.979) and end-of-exercise RPE (CAP: 19±0.8 vs. PLA: 18±2.4, ES: 0.89, p=0.623). In conclusion, acute CAP supplementation did not increase time to exhaustion during high-intensity continuous exercise nor alter physiological responses in runners.