Single-breath O2 consumption (VO2) at the mouth and heart rate were determined in five healthy male subjects at rest, during 8 min of cycloergometric exercise (50, 100, 125, and 150 W), and in the recovery period following two experimental conditions: air breathing throughout (AA); hypoxic breathing (FIO2 = 0.11) for 6 min of preexercise rest followed by air breathing from the onset of exercise (HA). The O2 deficits and debts as well as the t 1/2 values of the VO2 on- and off-responses were determined and blood lactate concentrations measured at rest and in the recovery after 4 and 8 min of exercise. At all work loads: 1) O2 deficits were on the average 0.39 liter smaller in HA than in AA; 2) VO2 on-responses were faster in HA (t 1/2 approximately equal to 7 s) than in AA (t 1/2 = 20-30 s); and 3) O2 debts and VO2 off-responses were the same in the two conditions. Since the VO2 and heart rate levels at steady state as well as the blood lactate concentrations after 4 and 8 min of exercise were the same in AA and HA, the observed differences of O2 deficit cannot be attributed to changes of energy metabolism in the two conditions; they therefore depend on the reduction of body O2 stores at rest in HA. This, independently measured, was found to be 0.46 liters, not far from the observed O2 deficit difference (0.39 liters). Thus a decrease of O2 stores before exercise is accompanied by a reduction of the O2 deficit and faster VO2 kinetics at the onset of exercise.