Little is known about maturation of peripheral chemoreceptor tone (PCT) during growth. We recently demonstrated that the increase in PCT was 49% greater during hypoxic (15% O2) exercise in children compared to adults. As the PCT is a major determinant of ventilatory (VE) response at the onset of exercise (measured by the time constant tau), we hypothesized that hypoxia would affect tau VE (and tau VCO2) to a greater extent in children. Nine healthy children (6-10 y old) and nine healthy adults (18-40 y old) performed multiple transitions from rest to constant work rate on the cycle ergometer. Studies were done breathing 21% O2 and 15% O2. Hypoxic breathing quickened the VE responses in all of the adults and children, but the magnitude of the hypoxic effect did not differ between the two groups (in children, tau VE was 50.9 +/- 9.9 s during 21% O2 breathing and 32.6 +/- 6.9 s during hypoxia; in adults, tau VE was 69.4 +/- 17.6 s, which fell to 50.9 +/- 18.4 s during hypoxia). The hypothesized greater ventilatory response to hypoxia in children compared to adults during exercise was not observed. During 21% O2 breathing, the data demonstrated that children stored relatively less CO2 (by 49%) than did adults in the transition between rest and exercise, possibly explaining the faster ventilatory kinetics. We speculate that there must be additional respiratory control differences between adults and children such that for a given increase in PCT-induced by hypoxia, the VE response at the onset of exercise is less in children than in adults.