The objective of this study was to verify the effect of the exercise mode on slow component of VO₂ (VO₂SC) in children aged 11–12 years during severe-intensity exercise. After determination of the lactate threshold (LT) and peak VO₂ (VO₂peak) in both cycling (CE) and running exercise (TR), fourteen active boys completed a series of “square-wave” transitions of 6-min duration at 75%∆ [75%∆ = LT + 0.75 × (VO₂peak—LT)] to determine the VO₂ kinetics. The VO₂SC was significantly higher in CE (180.5 ± 155.8 ml • min⁻¹) than in TR (113.0 ± 84.2 ml · min⁻¹). We can conclude that, although a VO₂SC does indeed develop during TR in children, its magnitude is considerably lower than in CE during severe-intensity exercise.