Although health initiatives promote increased physical activity in children, the physiologic outcomes have not been well characterized. This investigation examined the effects of programmed aerobic exercise on protein metabolism in children (n = 7; mean +/- SEM: 9.14 +/- 0.46 y old; weight, 32.1 +/- 1.6 kg; height, 138 +/- 2.5 cm; and body mass index, 16.21 +/- 0.36 kg/m(2) ) using (15)N-glycine methodology. Boys (n = 5) and girls (n = 2) walked (5 d/wk, 3.2-6.4 km/d) for 6 wk. Criterion measures taken at baseline (Pre) and after the exercise program (Post) included anthropometric data, dietary assessment, nitrogen balance, nitrogen flux (Q), protein synthesis (PS), protein breakdown (PB) and net protein balance [(Net) = PS - PB]. After the walking program, there were no significant changes in body weight, fat-free mass or percentage of body fat, whereas height increased (P < 0.01). Energy and protein intakes were constant throughout the study. Nitrogen balance was significantly more positive Post than Pre (P < 0.05). There was a significant decrease in Q (P < 0.0001) with corresponding decreases in PS (P < 0.001) and PB (P < 0.01). These data provide the first evidence that programmed aerobic exercise alters whole-body protein utilization in healthy, nonobese children. Longitudinal studies are required to further examine changes in protein metabolism associated with increased physical activity in this population. In addition, findings suggest a need to evaluate nutrient requirements for healthy, physically active boys and girls.