Oxygen uptake and heart rate responses during hypoxic exercise in children and adults. Control of ventilation and heart rate during exercise appears to undergo maturation, while aerobic metabolism (VO2) may not. Since we had previously found that hypoxia during exercise produced different ventilatory responses in children (C) compared to adults (A), we hypothesized that VO2 and heart rate kinetics during exercise would show similar maturational responses to hypoxia. To test this hypothesis, we examined the responses during progressive (ramp) and constant work rate tests in children and adults breathing either room air or hypoxic gas (FiO2 = 0.15). When corrected for body weight, children and adults had similar values for lactic acidosis threshold (LAT) (C: 29.1 +/- 5.0 ml.min-1.kg-1; A: 27.9 +/- 4.3) and VO2max (C: 40.7 +/- 8.6 ml.min-1.kg-1; A: 45.2 +/- 6.7) during normoxia. Hypoxia significantly lowered LAT (C: 27.5 +/- 5.4 ml.min-1.kg-1; A: 23.2 +/- 3.8; both P less than 0.05) and VO2max (C: 37.7 +/- 8.3 ml.min-1.kg-1; A: 40.1 +/- 5.3; both P less than 0.05) in both children and adults. Metabolic efficiency (delta VO2/delta work rate) and the VO2-heart rate relationship (delta VO2/delta HR/kg) were similar in the two groups and unaffected by hypoxia. During the constant work rate exercise, VO2 kinetics (time constant during phase 2 of the response (pi 1) and the O2 deficit) were similar between children and adults and were significantly slowed by hypoxia, consistent with current understanding of the control of oxidative metabolism. Finally, heart rate was increased at rest and during exercise with hypoxia, while the time to reach 75% of the end-exercise response was delayed significantly, in both groups. 