ABSTRACT. To test the hypothesis that ventilatory responses to exercise mature during growth in healthy children, we examined C 0 2 production (VCO~) and minute ventilation (VE) before, during, and for 10 min after 1-min bursts of cycle ergometry exercise. Ten children (range: 7-11 y old) and 13 adults (26-42 y old) exercised at work rates corresponding to 50 and 80% of the anaerobic or lactate threshold, 50% of the difference between anaerobic threshold and maximum 0 2 consumption, 100% of maximum O2 consumption, and 125% of maximum 0 2 consumption (125% max). Gas exchange was measured breath by breath. Children recovered faster from high-intensity (above anaerobic threshold) exercise as judged by the time constant of single exponential curve-fits to postexercise Vco2 (55 f 10 s (1 SD) at 125%. max in children compared with 92 2 17 s at 125% max in adults; p < 0.0011 and VE (58 f 10 s at 125% max in children compared with 125 & 37 s in adults, p c 0:001). Although we found no significant difference between VCOZ and VE recovery times in children, VE was significantly slower than Vco2 in adults for highintensity exercise. Moreover, recovery times in adults increased with work intensity but were independent of them in children. Whereas the C 0 2 costs [calculated as total C 0 2 produced above baseline per unit work done (mL. J-I)] increased with work intensity in adults, no similar significant relationship was observed in children. Finally, at 125% max, the ratio of C 0 2 cost to O2 cost was 1.48 f 0.07, less than the ratio in adults (1.83 -C 0.07, p < 0.01).These results demonstrate marked differences in dynamic ventilatory response to short bursts of exercise reflecting maturation of both respiratory control and cellular metabolic factors. (Pediatr Res 29: 362-368, 1991) Abbreviations AT, anaerobic threshold VCOZ, C 0 2 production VE, minute ventilation VO~, 0 2 consumption Petco2, end-tidal Pco2 Vo2max, maximum O2. uptake A, difference between V02max and AT % max, % of Vo2max 7Vco2, time constant for V C O~ recovery 7VE, time constant for Ve recovery Ventilation in humans is regulated to maintain cellular homeostasis of COz and bicarbonate when the metabolic rate changes. The link between cellular activity and ventilation is, in large part, dependent on the distribution and transport of .COz molecules (l,2). There is evidence that the linkage between Vco2 and VE undergoes a process of maturation during growth, i.e. there is a difference between children and adults in the ventilatory response to changes in metabolic rate (3). The magnitude and mechanisms of these differences have not been fully elucidated, most likely because studying gas exchange in ways that are acceptable to children is difficult and because there are no uniform approaches to normalizing physiologic responses in subjects of widely different sizes. In this study, we employed an exercise protocol specifically designed for the behavioral characteristics of children, and the data were normalized and analyzed in a manner that facilitated compar...