Improving fat oxidation rate (FOR) may have positive implications for weight management. It has been suggested that exercise training at the intensity (Fatmax) that elicits maximal fat oxidation rate (MFO) can improve FOR in obese adults and adolescents. There have been relatively few Fatmax studies of children, and the effect of sex and maturation is not well-established. PURPOSE: To investigate the effect of sex and pubertal maturation on MFO and Fatmax in children. METHODS: Boys (n=13) and girls (n=18) volunteered for this study. Boys were pubertal stage 1-2 (YB, n=9) and ≥3 (OB, n=4) according to Tanner. Girls were premenarcheal (YG, n=12) and menarcheal (OG, n=6). Subjects performed a Fatmax test on a cycle ergometer followed by a maximal exercise test. FORs were calculated for each stage of the Fatmax test and were graphed against exercise intensity. A best-fit polynomial curve was applied to the data. MFO was interpolated as the peak of the curve, and the corresponding exercise intensity was deemed Fatmax. Effects of maturation and sex on Fatmax and MFO were evaluated, and specific group differences were isolated. RESULTS: Absolute MFO was 0.18±0.08 g/min, 0.15±0.04 g/min, 0.14±0.06 g/min, and 0.18±0.06 g/min in the YB, YG, OB, and OG groups, respectively. MFO relative to body mass (BM) was 5.6±2.7 mg/kg/min, 4.8±1.7 mg/kg/min, 2.7±1.0 mg/kg/min, and 3.3±1.1 mg/kg/min in the YB, YG, OB, and OG groups, respectively. MFO relative to fat-free mass (FFM) was 7.2±3.6 mg/kgFFM/min, 6.7±2.2 mg/kgFFM/min, 3.2±1.2 mg/kgFFM/min, and 4.9±1.6 mg/kgFFM/min in the YB, YG, OB, and OG groups, respectively. Fatmax was 45±11%, 43±10%, 36±6%, and 38±5% of VO2max in the YB, YG, OB, and OG groups, respectively. There was no sex effect for any of the four metrics, or a maturation effect for absolute MFO. There was a significant maturation effect on MFO relative to BM and on MFO relative to FFM, and a maturation effect approached significance for Fatmax. The less-mature groups had significantly higher MFO relative to BM and FFM than had the more-mature groups, with a trend toward a higher Fatmax. CONCLUSIONS: The results suggest that sex may not have an effect on MFO or Fatmax in children, but that there is likely a maturation influence. Funded by the Graduate Research Award from the Midwest Chapter of the American College of Sports Medicine.