It is unclear whether hypoxia alters the kinetics of O2 uptake (VO2) during heavy exercise [above the lactic acidosis threshold (LAT)] and how these alterations might be linked to the rise in blood lactate. Eight healthy volunteers performed transitions from unloaded cycling to the same absolute heavy work rate for 8 min while breathing one of three inspired O2 concentrations: 21% (room air), 15% (mild hypoxia), and 12% (moderate hypoxia). Breathing 12% O2 slowed the time constant but did not affect the amplitude of the primary rise in VO2 (period of first 2-3 min of exercise) and had no significant effect on either the time constant or the amplitude of the slow VO2 component (beginning 2-3 min into exercise). Baseline heart rate was elevated in proportion to the severity of the hypoxia, but the amplitude and kinetics of increase during exercise and in recovery were unaffected by level of inspired O2. We conclude that the predominant effect of hypoxia during heavy exercise is on the early energetics as a slowed time constant for VO2 and an additional anaerobic contribution. However, the sum total of the processes representing the slow component of VO2 is unaffected.