The aim of the study was to examine the influence of the respiratory insufficiency in infants treated conservatively or with the use of mechanical lung ventilation, on cardiovascular activity of the autonomic nervous system and interdependence of the sympathic and parasympathic system in infants. The cardiovascular autonomic nervous system functions were estimated by the power spectrum analysis of the heart rate variability (PS/HRV) using MEDEA HRV system. HRV spectrum analysis was limited to low (LF: 0.05-0.2 Hz) and high (HF: 0.2-0.6 Hz) frequency components, which were indicators of the sinoatrial node modulation by sympathic (LF) or parasympathic (HF) nervous system. LF/HF ratio presents the independence of the sympathic and parasympathic nervous system activity, and the total power spectrum of the HRV is the whole vegetative system activity index. The medical research was carried out in 44 infants. They included 29 infants with respiratory insufficiency conservatively treated in intensive care unit and 15 healthy control subjects. All infants with respiratory insufficiency were examined twice: during respiratory insufficiency and after recovery (40-64 days from first examination). Healthy infants were examined only once. According to the HRV parameters analysis it was proved that in infants respiratory insufficiency has significant influence on the increased cardiovascular activity of the parasympathic system, and increased cardiovascular activity of the autonomic nervous system. Respiratory insufficiency significantly changes relationship between cardiovascular activity of the sympathic and parasympathic nervous system. Regression of respiratory insufficiency symptoms causes the normalisation of both sympathic and parasympathic activity.