Aim of the study was to determine dynamics of main parameters of heart rhythm in children during first days of life. Material and methods. We examined 30 healthy newborns (14 boys and 16 girls) aged 1-4 days (2,6+/-1,3 days). Criteria of inclusion were healthy mother, normal course of pregnancy and delivery, gestational age at birth 38-40 weeks, body mass at birth 3200-3600 g; Algar score 8/9, no cardiovascular abnormalities at physical examination, informed consent of parents. On days 1-2 and 4 of life we carried out 12 lead ECG at rest and 24-hour Holter monitoring with assessment of heart rate (HR), arrhythmias, dynamics of QT-interval, HR variability (HRV) using methods "Time Domain" and "deceleration (DC)/acceleration capacity (AC)". Results. Average 24-hour HR was 138+/-17 beats/min, maximal pauses of sinus rhythm - 990 ms. Supraventricular extrasystoles (<10/24 hours) were registered in 35%, ventricular extrasystoles (<5/24 hours) in 15% of children. Values of HRV were as follows: N 442 (408-72) ms; SDNN 52 (43-75) ms; SDNNi 36 (35-61) ms; SDNNi24 (22-37) ms; SSD 14 (10-20) ms; NN50% 0,98(0-2,4)%. Analysis of D/ showed the following values: D = +3.43 +/- 0.54 (2.16-4.14),=-3.65+/-0.65 (from - 2.13 to - 4.38). Analysis of interval QT values: average 24-hour QT (QTo) - 293+/-10 (278-308) ms, maximal - 342+/-38 (328-360) ms; mean 24-hour QT 442+/-14 (428-470) ms; QT 204+/-11 (191-220) ms; QT - 308+/-17(284-337) ms, sl Q/RR 0.37 (0.304-0.505); intercept QT/RR 126 (92.0-151.0), correlation coefficient () QT/RR 0.65 (0.56-0.84). On day 4 compared with day 1 there were significantly higher values of correlation (r) QT/RR and lower values of sl QT/RR. Conclusions. During first days of life children can have supraventricular and ventricular extrasystoles (not more than 10 per 24 hours). First 24 hours of life are characterized by most pronounced signs of myocardial instability according to data of assessment of QT dynamics. HRV parameters of healthy newborns are characterized by high level of sympathetic activity, symmetry of deceleration capacity/acceleration capacity parameters.