Although the heart rate variability (HRV) values in adults decrease with aging, those in children show a variety of changes. The present study was designed to investigate the relationship between HRV and aging, physique and blood pressure in 70 healthy male school children, between 6 and 12 years of age. The subjects were divided into 3 age groups (6-7, 8-9 and 10-12 years) and 3 other groups according to physique (thin, under -10%; normal, -10-20% and obese, over 20% of obesity index). After recording a 24-h ambulatory electrocardiograph, HRV was determined spectrally, using 3 components: the total-frequency (TF) component (0.01 to 0.5 Hz), the low-frequency (LF) component (0.04 to 0.15 Hz), the high-frequency (HF) component (0.15 to 0.5 Hz) and also a two component ratio (LF/HF). The HRV values between the 3 age groups had a significant effect on TF, LF and LF/HF during the 24-h period, with the values increasing with age. The HRV values between the 3 physique groups had no significant effect. Therefore, changes in HRV in school children are affected by age and not physique. It is possible that the increase in HRV values with age is caused by the development of the autonomic nervous system.