UNLABELLED: Exercise training results in a speeding of pulmonary oxygen uptake (VO2) kinetics at the onset of exercise in adults; however, only limited research has been conducted with children and adolescents. PURPOSE: The aim of the present study was to examine VO2 and muscle deoxygenation kinetics in trained and untrained male adolescents. METHODS: Sixteen trained (15 0.8 yr, VO2peak = 54.7 6.2 mL x kg-1 x min-1, self-assessed Tanner stage range 2-4) and nine untrained (15 0.6 yr, VO2peak = 43.1 5.2 mL x kg-1 x min-1, Tanner stage range 2-4) male adolescents performed two 6-min exercise transitions from a 3-min baseline of 10 W to a workload equivalent to 80% lactate threshold separated by a minimum of 1 h of passive rest. Oxygen uptake (breath-by-breath) and muscle deoxygenation (deoxyhemoglobin signal from near-infrared spectroscopy) were measured continuously throughout baseline and exercise transition. RESULTS: The time constant of the fundamental phase of VO2 kinetics was significantly faster in trained versus untrained subjects (trained: 22.3 7.2 s vs untrained: 29.8 8.4 s, P = 0.03). In contrast, neither the time constant (trained: 9.7 2.9 s vs untrained: 10.1 3.4 s, P = 0.78) nor the mean response time (trained: 17.4 2.5 s vs untrained: 18.3 2.3 s, P = 0.39) of muscle deoxygenation kinetics differed with training status. CONCLUSIONS: The present data suggest that exercise training results in faster VO2 kinetics in male adolescents, although inherent capabilities cannot be ruled out. Because muscle deoxygenation kinetics were unchanged, it is likely that faster VO2 kinetics were due to adaptations to both the cardiovascular system and the peripheral musculature.