Heart rate variability is a noninvasive index of the neural activity of the heart. The present study examined heart rate variability indices in 210 infants and children aged 3 days to 14 years to obtain normal ranges for all age classes. Heart rate variability was measured by calculating mean RR interval over the length of the analysis, mean RR interval during quiet sleep, 5 time-domain (SDNN, SDNN-i, SDANN-i, r-MSSD, pNN50), and 4 frequency-domain (VLF, LF, HF, LF/HF ratio) indices. Our data show a significant positive correlation between all indices and the mean RR interval over the length of the analysis, except for the LF/HF ratio for which the correlation was binomial. A positive power correlation was also found between all parameters and age. The multiple correlation analysis confirmed the independent effect of age and mean RR interval on the heart rate variability. These data in a healthy pediatric population confirm a progressive maturation of the autonomic nervous system during childhood and may be utilized to examine the effects of underlying disease processes or therapeutic interventions on cardiac autonomic tone during infancy and childhood.