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The purpose of this study was to investigate longitudinal changes with age in the kinetic response to cycling at heavy-intensity exercise in boys and girls. Twenty-two prepubertal children (13 male, 9 female) carried out a series of exercise tests on two test occasions with a 2-yr interval. On each test occasion, the subject completed multiple transitions from baseline to 40% of the difference between their previously determined V-slope and peak O(2) uptake (Vo(2)) for 9 min on an electronically braked cycle ergometer. Each subject's breath-by-breath responses were interpolated to 1-s intervals, time aligned, and averaged. The data after phase 1 were fit with 1) a double exponential model and 2) a single exponential model within a fitting window that was previously identified to exclude the slow component. There were no significant differences in the parameters of the primary component between each model. Subsequent analysis was carried out using model 2. The Vo(2) slow component was computed as the difference between the amplitude of the primary component and the end-exercise Vo(2) and was expressed as the percent contribution to the total change in Vo(2). Over the 2-yr period, the primary time constant (boys 16.8 +/- 5.3 and 21.7 +/- 5.3 s, girls 21.1 +/- 8.1 and 26.4 +/- 8.4 s, first and second occasion, respectively) and the relative amplitude of the slow component (boys 9.4 +/- 4.6 and 13.8 +/- 5.3%, girls 10.3 +/- 2.4 and 15.5 +/- 2.8%, first and second occasion, respectively) significantly increased with no sex differences. The data demonstrate that children do display a slow-component response to exercise and are consistent with an age-dependent change in the muscles' potential for O(2) utilization.