Intense aerobic exercise has been found to prompt changes in oxidative stress, but in children remains almost unexplored. The aim was to investigate the effect of intense physical exercise on reduced glutathione (GSH as a biomarker of oxidative stress) and adrenocortical response (to verify a certain level of stress after exercise) in 38 prepubescent and 32 pubescent non-athlete boys. Four subgroups were established as puberty stage and physical fitness. Saliva samples were taken before and after incremental exercise testing to measure GSH, and cortisol levels. Saliva reduced glutathione levels were lower in all subgroups after exercise except in the prepubescent average fit group, significance being greater in the pubescent (P < 0.001) than in the prepubescent group (P < 0.01). Saliva cortisol increased after exercise in all except in the prepubescent "average fit" group. Physical exercise may give rise to oxidative stress and adrenocortical response in pubescent and prepubescent boys, depending on the duration and intensity of the test.