Multifactorial studies were performed in child and adolescent swimmers in the early period of training and in a submaximum state of training in the year's racing period. Blood coagulation, viscosity, acidity, protein metabolism, muscle enzymes, ions, haematologic, cardiac and endocrine parameters have been investigated. In the early period of training, physical exercise resulted in fibrinolysis, and a decrease in the fibrinogen and prothrombin levels and the platelet count. The same exercise in fully-trained organisms failed to produce such changes. Data concerning acidity, protein metabolism, muscle enzymes, ions, haematologic, cardiac and endocrine parameters will be published in subsequent papers.