Little information is available on energy metabolism during exercise in girls, particularly the contribution of exogenous carbohydrate (CHO_exo). The purpose of this study was to determine substrate utilization during exercise with and without CHO_exo intake in healthy girls. Twelve-yr-old (YG; n=12) and 14-yr-old (OG; n=10) girls consumed flavoured water (WT) or ¹³C-enriched 6% CHO (CT) while cycling for 60 min @ ~70% V·O_2max. Substrate utilization was calculated for the final 15 min of exercise. CHO_exo decreased fat oxidation by ~50% in YG, but not in OG (P < 0.001) and decreased endogenous CHO oxidation by ~15% in OG, but not in YG (P = 0.006). Endogenous CHO oxidation was lower in YG than in OG regardless of trial (P 0.01), whereas fat oxidation was higher in YG only during WT (P < 0.001). CHO_exo oxidation rate was similar between YG and OG (7.1 ± 0.5 and 6.8 ± 0.4 mg·kg^-1·min^-1, respectively, P = 0.67) contributing ~19% to total energy expenditure. Serum estradiol levels in all girls correlated with fat (r = -0.50 to -0.59, P = 0.03 to 0.005) and CHO_endo (r = 0.50 to 0.63, P = 0.03 to 0.005) oxidation, but not with CHO_exo oxidation (r = -0.09, P = 0.71). We conclude that CHO_exo influences endogenous substrate utilization in an age-dependent manner in healthy girls, but that total CHO_exo oxidation during exercise is not different between YG and OG. Our results also point to potential sex-related differences in energy substrate utilization even during childhood.