In the study 26 trained male and female sportsmen aged 18-20 years old have been investigated. The sex differences in the degree of the nitric oxide synthesis in various stages of their training and competitive activities were shown. In female sports­men, at the time of maximal functional capacity and adapta­tion (beginning of the competition period), a multi-path nonoxidizing arginine metabolism by arginase was prevailed over the oxidative degradation of L-arginine by cNOS . In females, during dezadaptation period (in the middle and the end of competition), the required level of NO synthesis is maintained mainly by non-oxidative resynthesis of nitric ox­ide from its stable metabolites. In males, however, the level of NO synthesis is maintained mainly by an increase in oxidative degradation of L-arginine with the gradual change in activity of different isoforms of NOS (cNOS as well as iNOS).