The hormonal response to a short but intense session of physical exercise should be distinguished from the endocrine adaptation to systematic physical conditioning. The normal child is perfectly equipped to handle stress situations such as those generated by leisure sport: the transient increase in stress hormones has no deleterious effect on growth and puberty. For highly trained children and adolescents, standardized dynamic testing will provide little information on the state of their endocrine system. In addition, the effects of training should be differentiated from those of various bias of selection. In young elite athletes, as for adult athletes, the alterations of the endocrine system result from an inappropriate physical conditioning programme for the individual level of tolerance. Whereas it has been shown that the gonadal function is predominantly affected (pubertal delay, menstrual dysfunction), alterations of growth hormone and cortisol productions have also been reported. Anomalies of pubertal growth should be searched for among elite adolescent athletes: there are data suggesting that their growth potential should probably not be affected below 15 weekly hours of training. However, children do not respond to stress in a uniform manner and one should be prepared to detect the occasional athlete with inadequate growth at lower training intensities. This seems to be the case when training starts before puberty as well as in physical activities associated with a strict weight control. When growth and/or puberal progression become inappropriate, the only logical therapy consists in reducing markedly or stopping training temporarily: in this situation, there is no medical justification whatsoever to initiate a substitutive therapy.