INTRODUCTION. The leading role in the water and electrolyte balance in critical conditions belongs to infusion therapy. Hemodynamic stabilization is achieved by fluid bolus with risk of hypervolemia. Non-invasive monitoring in ICU is relevant in children. Cardiointervalography is characterized by nosological non-specificity and high sensitivity to neurohumoral changes. The stress index and spectral analysis allow the assessment of therapeutic efficacy. OBJECTIVES. To study the features of the electrophysiological response to the emergency rehydrate volume in children upon admission to the intensive care unit. MATERIALS AND METHODS. A prospective comparative study was conducted with the participation of 172 patients, the age from 0 to 15 years. There were 71 newborns in the first group (21 surgical cases, 50 somatic ones). The second group enrolled children aged 1 to 15 years, in 40 % of all underwent surgical treatment. The characteristic of the electrophysiological response to the hydration was carried out in an intergroup and interstage comparison in different age and nosological groups. DISCUSSION. The autonomic nervous system response to infusion therapy has not been studied in children of different ages. The stress index and frequency analysis reflect the degree of regulatory systems tension and are promising in intensive care monitoring. RESULTS. In newborns with sensory block, it was expressed by tachycardia (p = 0.02); without block - arterial hypotension (p = 0.000) without chronotropic component. The implementation of sympathetic reactions was suppressed in children in the early postoperative period. In children without surgical treatment, the volume of infusion had a sympatholytic effect. CONCLUSIONS. In the early postoperative period, the electrophysiological response in children was limited. Age-related features of reactions to hydration in children with somatic pathology were revealed. In children, infusion therapy contributed to a cardiovascular economical work; in newborns, it was associated with a breakdown in sympathetic adaptation.