BACKGROUND: Heart rate variability analysis is a modern, non-invasive, precise and sensitive method of the assessment of cardiovascular systemic functions. OBJECTIVES: Find out changes in parameters of time-domain analysis of heart rate variability (HRV) in children after cardiac transplantation. METHODS: Twenty-two subjects were enrolled and examined in this study--2 subjects after cardiac transplantation (14 and 5 years of age) and 2 control groups of healthy subjects (10 subjects at age from 14 to 16 and 10 subjects at age from 5 to 7 y). Five parameters of HRV from 24-hour Holter monitoring have been analysed (NN intervals, SD, pNN50, rMSSD, triangular index). GiOtto ECG Holter monitor (Medatron) was used in each child. Time domain analysis parameters have been controlled and compared by using percentile range obtained from healthy individuals (Pl0-P90). RESULTS: rMSSD, pNN50, triangular index were below 10th percentile in a 14-year-old girl. rMSSD, SD were below the 10th percentile in a 5-year-old boy. CONCLUSION: Total heart rate variability (described by triangular index and SD parameter) and parameters describing parasympathetic activity (pNN50 and rMSSD) were decreased in both patients after cardiac transplantation. Considering the still unclear mechanism of heart innervation, it is necessary to continue in the research. Authors are planning to check the changes in the future also by means of frequency analysis of heart rate variability. (Tab. 3, Ref. 22.)