Insulin resistance and glucose tolerance play a central role in the development of type 2 diabetes and cardiovascular disease, the origin of which may reside in childhood. Interventions that improve glucose tolerance and insulin sensitivity in youth may therefore have a protective role on future health. Evidence relating to the health benefit of regular physical activity is accumulating, yet few adolescents meet current recommendations and knowledge of the optimum “dose” is lacking. The purpose of this study was to examine whether a single bout of time efficient, high-intensity interval exercise (HIIE) was superior to moderate intensity exercise (MIE) at improving glucose tolerance, insulin sensitivity (IS) and resting fat oxidation in adolescent boys. Nine normal weight, healthy participants (age: 14.2 ±0.4 y; VO2 max: 46.5 ± 9.6 mL/kg/min) completed three one day trials, with all exercise performed on a cycle ergometer: 1) HIIE; 8 x 1 min at 90% of peak power with 75 s recovery; 2) MIE; 90% of gas exchange threshold, the duration of which was determined to match the work done during HIIE, and 3) rest in the laboratory (CON). Exercise enjoyment was assessed after each exercise type. Ten min after exercise or CON an oral glucose tolerance test was performed (75 g glucose in 300 mL water) with plasma [glucose] and [insulin] measured at 0, 10, 20, 30, 60, 90 and 120 min post glucose load. Fat oxidation was measured (indirect calorimetry) at hourly intervals for 3 h. IS was calculated using the Cederholm index. Differences between conditions were assessed using repeated measures ANOVA and subsequent pair wise comparisons were interpreted using probability (P) and effect sizes (ES) with the smallest worthwhile change taken as an ES>0.2.The incremental AUC (iAUC) for plasma [glucose] was lower after MIE (-23.9%, P=0.013, ES=-0.64) and HIIE (-28.9%, P=0.008, ES=-0.84) compared to CON. HIIE reduced iAUC plasma [glucose] by 6.6% compared to MIE (P=0.22, ES=-0.24).The iAUC for plasma [insulin] was lower for HIIE (-24.2%, P=0.021, ES=-0.71) and MIE (-29.1%, P= 0.012, ES= -0.79) compared to CON. IS increased by 8.4% after MIE (P=0.10, ES=0.58) and 11.2% after HIIE (P=0.03, ES=0.76). The total AUC for fat oxidation was higher after HIIE compared to CON (19.8%, P=0.28, ES=0.43) and MIE (27.8%, P=0.15, ES=0.58). HIIE and MIE were equally enjoyable (P= 1.0, ES<0.01). Although both MIE and HIIE are effective at improving glucose tolerance and IS in adolescent boys, the effects of HIIE were superior in terms of enhancing glucose tolerance and postprandial fat oxidation. HIIE is equally enjoyable as MIE and more time efficient, highlighting its potential as a feasible and effective form of exercise to improve metabolic health outcomes in adolescents.