Muscle O₂ saturation (SmO₂) and blood volume response in activating muscles during treadmill exercise were compared between prepubertal boys (n = 9, age: 9 ± 1 years) and young men (n = 9, age: 22 ± 2 years). SmO₂ and blood volume responses were monitored continuously during the exercise at the gastrocnemius medialis muscle by near infrared spatial resolved spectroscopy. SmO₂ was significantly decreased only at peak exercise in the boys, even though a significant decrease in SmO₂ was observed at 60, 80, and 100 % of peak O₂ uptake in the men (p < 0.05). No significant increase in blood volume was observed in the boys, while blood volume was significantly increased in the men with increased exercise intensity (p < 0.05). These results suggest that both blood volume and deoxygenation response in activating muscle may be minor in prepubertal boys, compared to young men. The blunted deoxygenation response in prepubertal boys may be caused by undeveloped diffusive O₂ transport (i.e. mitochondrial respiration).