Introduction: Training in the cadet corps involves an increase in motor activity, which affects the level of functional capabilities of adaptation systems. The objective of our work was to study the features of functional reserves of the cardiorespiratory system in cadets. Materials and methods: The observation group consisted of 74 cadets aged 10–13 years; the comparison group consisted of 26 secondary school boys. The study of the mode of physical activity was carried out using a questionnaire. Evaluation of fitness and physical performance was carried out according to bioimpedance analysis of body composition and dynamometry. The functional state of the cardiorespiratory system was assessed by the parameters of the breath holding test during inhalation and exhalation, the Skibinskaya index, spirography, cardiointervalography with an active clinoorthostatic test. Statistical analysis was performed using standard methods. Results: Every second cadet regularly goes in for physical education and sports. The physical development of cadets is characterized by higher values of the phase angle, active cellular and musculoskeletal mass, and dynamometry. The functional reserves of the cardiorespiratory system according to the Skibinskaya index, the values of the breath holding time during inhalation and exhalation in cadets were 1.3–1.7 times higher than those in secondary school boys; the number of children with good test results in the observation group was 1.4-3.3 times more than that in the controls. The indices of cardiointervalography (variation range, vegetative balance index, vegetative balance index, and tension index) in cadets differed significantly from those in the comparison group (by 1.2–1.6 times) and indicated the activation of the parasympathetic part of the autonomic nervous system at rest and the activity of the sympathetic part under load in response to a stress factor. Every third child in the observation group had an initial vagotonia; hypersympathicotonia was found in isolated cases. A hypersympathicotonic variant of autonomic reactivity was registered in 60.8 % of cadets during the exercise test. Conclusion: Sufficient physical activity of cadets improves the indices of physical development and muscle strength, which is accompanied by an increase in the functional reserves of the respiratory and cardiovascular systems. However, the presence of hypersympathicotonic autonomic reactivity may indicate tension of adaptation processes during exercise.