Heart rate variability (HRV) is a noninvasive measure of autonomic input to heart rate that has been successfully used to estimate modulation of autonomic tone. The authors investigated to determine the normal ranges as a standardized tests of heart rate variability (HRV) for autonomic function tests in a large sample of healthy Koreans. The study also evaluated the effects of age, sex, and heart rate (HR) on the results of HRV and the values of HRV in hypertension, diabetes, and obesity group. Six hundred and thirty-seven healthy subjects who visited the health improvement center for medical examination were evaluated. A standardized 5-min resting study, including spectral analysis of HR, was used to all participants after an overnight fast. HRV (SA-2000E, Medicore, Korea) was spectrally determined by three components: total-frequency (TF) component (0.01 to 0.5 Hz), low-frequency (LF) component (0.04 to 0.15 Hz), and high-frequency (HF) component (0.15 to 0.5 Hz). Also two component ratio (LF/HF) was calculated. Routine laboratory and physical examinations were used to eliminate cardiovascular diseases and neurological disorders. The determinants of short-period HRV in a random sample of 366 men and 271 women from healthy subjects were assessed. The mean age of these subjects was 45.1 +/- 10.7 years. The mean value of total power was 1106.9 +/- 1109.1 ms(2); SDNN was 35.9 +/- 15.5 ms; rMSSD was 27.3 +/- 15.6 ms for time domain analysis. The mean value of LF was 287.5 +/- 384.1 ms(2); HF was 227.0 +/- 284.4 ms(2); LF/HF was 2.2 +/- 3.4 for frequency domain analysis. Additionally, men showed that their TP, LF, and LF/HF values were significantly higher than women whereas SDNN, rMSSD, HF showed no significant differences between sexes. With increasing age, there was no significant decrease in HR. TP, SDNN, LF, and HF were significantly decreased when getting older. On the contrary, LF/HF had no significant difference related to age. The study confirmed that SDNN and rMSSD were significantly lower in hypertension and diabetes group. In conclusion, middle-aged men had more pronounced sympathetic influence than women in cardiac regulation, and HRV declined linearly with age. In addition, HRV reflecting the function of autonomic nervous system was decreased in hypertension and diabetes group.