To test the hypothesis that glycolytic metabolism in muscle is attenuated in prepubertal children, ³¹P-magnetic resonance spectroscopy was used to determine calf muscle intracellular pH (pH_i) in nine prepubertal (Pre) and nine pubertal female swimmers (Pub). Maximal plantar flexion work capacity (100% MWC) was established by using a graded exercise test. Between 5 and 10 days later, calf muscle images (magnetic resonance imaging) and phosphorus spectra were acquired at rest, during 2 min of light exercise (40% MWC), and during 2 min of supramaximal exercise (140% MWC) in a 3.0-T NMR system. End-exercise pH_i was 6.66 ± 0.11 and 6.76 ± 0.17 for Pub and Pre, respectively. No significant differences in the mean values for pH_i or the P_i-to-phosphocreatine ratio were observed between groups during the protocol; however, an interaction effect was found for the P_i-to-phosphocreatine ratio during the supramaximal exercise challenge. Cross-sectional area of gastrocnemius was 15.12 ± 0.46 and 9.37 ± 0.37 cm² for Pub and Pre, respectively (P < 0.05). Differences in muscle size must be considered when interpreting the unlocalized magnetic resonance spectroscopy data. These results suggest that glycolytic metabolism in physically active children is not maturity dependent.