The metabolic responses to mixed breakfast meals with different glycemic indexes (GI) and their effects on substrate metabolism during exercise in adolescent girls have not been examined. The interaction with weight status also warrants investigation. This study investigated the effect of mixed breakfast meals containing high GI (HGI) or low GI (LGI) carbohydrates on metabolic responses and fat oxidation during rest and exercise in overweight (OW) and non-overweight (NO) adolescent girls.A total of 8 OW and 12 NO adolescent girls consumed an isoenergetic HGI (GI=73) or LGI (GI=44) breakfast 120 min before completing a 30-min treadmill walk at 50% O2peak.Peak blood glucose concentration was higher for HGI compared with LGI in OW (P=0.023), but not NO (P>0.05) girls. Blood glucose total area under the curve (TAUC) was 13% higher in HGI compared with LGI in OW (P=0.006), but only 4% higher in NO (P=0.072) girls. Plasma insulin data were loge transformed (lninsulin). Plasma lninsulin concentrations were not different between HGI and LGI (P>0.05). Peak plasma lninsulin concentration (P=0.016) and TAUC (P=0.001) were greater in OW than NO girls. Fat oxidation during postprandial rest and exercise was not different between breakfasts (P>0.05).The elevated glycemic response in HGI compared with LGI was more pronounced in OW girls, suggesting a reduced ability to cope with the metabolic demands of the HGI, but not LGI, breakfast. Manipulation of breakfast GI did not alter fat oxidation during rest or subsequent moderate intensity exercise in OW and NO adolescent girls.