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This study examined the influence of alterations in maximal oxygen uptake (V˙O2max) in mediating performance and pacing during prolonged self-paced exercise.Twelve well-trained cyclists completed a 750-kJ time trial in temperate (COOL, 18°C), hot (HOT, 35°C), and hypoxic (HYP, 18°C; FiO2, 0.145) conditions, creating models with a stable, progressively decreasing, and acutely decreased V˙O2max, respectively.Trial completion was faster in COOL (48.2 ± 5.7 min) compared with HOT (55.4 ± 5.0 min) and HYP (60.1 ± 6.5 min) (P < 0.001), with HOT being faster than HYP (P = 0.028). Core temperature reached 39.0°C ± 0.6°C (COOL), 39.8°C ± 0.5°C (HOT), and 38.5°C ± 0.4°C (HYP; P < 0.01). Power output during COOL was higher than HOT from 40% of work completed onward (P < 0.05) and for the entirety of HYP (P < 0.001), in which it was lower than HOT at 20%–30% (P < 0.05). Normalized power output during COOL and HYP varied by ∼13% and ∼16%, respectively, whereas a ∼27% variation occurred in HOT. V˙O2 in COOL was higher than HOT from 70% onward (P < 0.01) and higher than HYP throughout exercise (P < 0.001). Relative to baseline V˙O2max (%V˙O2max) in normoxia (COOL and HOT) and hypoxia (HYP), %V˙O2max during HOT (78% ± 8%) was lower than COOL (84% ± 7%; P = 0.005) and HYP (87% ± 5%; P = 0.003).Despite an acutely reduced V˙O2max and power output in HYP, pacing and %V˙O2max were similar to COOL. In contrast, the progressive decrease in V˙O2max and power output in HOT resulted in a more variable pacing pattern with %V˙O2max decreasing throughout exercise. These data support the premise that pacing is associated with maintaining an optimal performance intensity, in conjunction with acute and progressive alterations in V˙O2max.