The range of exercise intensities that elicit high fat oxidation rates (FOR) in youth and the influence of pubertal status on peak FOR are unknown. In a longitudinal design, we compared FOR over a range of exercise intensities in a small cohort of developing pre-pubertal males. Five boys all at Tanner stage 1 (ages 11-12 years) and nine men (ages 20-26 years) underwent an incremental cycle ergometry test to volitional exhaustion. FOR curves were determined from indirect calorimetry during the final 30 seconds of each increment. The same protocol was duplicated annually in the boys as they progressed through puberty. The peak FOR was considerably higher (P<0.05) in boys at Tanner I (8.6 ± 1.5 mg·kg lean body mass·min-1) (mean ± SD) compared with men (4.2 ± 1.1 mg·kg lean body mass·min-1). FOR dropped as boys developed through puberty (Tanner 2/3 peak rate= 7.6 ± 0.6 mg·kg lean body mass·min-1; Tanner 4 peak rate = 5.4 ± 1.8 mg·kg lean body mass·min-1, main effect of Tanner stage; P<0.05) to the levels found in men (NS). The exercise intensity that elicited peak FOR was higher in the boys at Tanner I (56 ± 6 % VO2peak) than in men (31 ± 4% VO2peak) (P<0.001). This value tended to decrease by Tanner stage 4 (45 ± 10% VO2peak, main effect of Tanner stage; P=0.06)]. We conclude that, compared to men, prepubertal boys have higher relative FOR throughout a wide range of exercise intensities and that FOR drops as boys develop through puberty