This study was aimed to elucidate the changes in heart rate variability during treadmill exercise at various speeds and grades by spectral analysis. Thirty-three untrained male college students aged 20 ~ 26 yr were employed to exercise on a treadmill using 4 speeds (4.02, 5.47, 6.76 and 8.05 km/h) and 6 grades (0, 4, 8, 12, 16 and 20%). A fixed speed was selected for each session with the grade increased every 3 min. The electrocardiogram, respiration and the stepping activity were continuously recorded through an A/D converter system on the computer disk. Power spectra of heart rate variability (RRV) were obtained by use of a fast Fourier transform algorithm. The frequency domain was divided into 3 bands: VLF (0 ~ 0.04 Hz), LF (0.04 ~ 0.15 Hz) and HF (0.15 ~ 1.00 Hz). Heart rate was 74.4±2.1 beats/min at rest and showed a steady increase during treadmill exercise with increasing speed and grade up to 196.7±5.0 beats/min. Total power of HRV was 35.0±6.7 (beats/min)² at rest and progressively decreased during exercise down to 1.9±0.3 (beats/min)². The %VLF power of HRV was 34.5±3.7 % at rest and showed no significant change during exercise except for a decrease observed at the highest intensity of exercise. The %LF power was 44.1±3.0 % at rest and showed a progressive decrease down to 4.5±1.0 % during those stages of exercise where heart rate was over 135 beats/min. The %HF power was 21.4±2.9 % at rest and showed a progressive increase up to 87.1±6.7 % during higher intensity exercise where heart rate was over 165 beats/min. Peak frequency of HF band was 0.200±0.018 Hz at rest and was shifted to higher frequencies up to 0.909±0.048 Hz at heart rates greater than 135 beats/min. Respiratory frequency was 18.0±1.5 breaths/min at rest and significantly increased during exercise up to 53.0±3.7 breaths/min. Stride frequency during treadmill exercise showed an increasing tendency with increasing speed from 55.6±0.9 steps/min at 4.02 km/h to 81.2±0.6 at 8.05 km/h. It was concluded that total power of HRV decreased progressively with increasing exercise intensity due to the withdrawal of parasympathetic activity. At higher exercise intensity, % LF power decreased and %HF power increased with its peak frequency shifted to higher values in a progressive mode with increasing speed and grade, reflecting a readjustment in the cardiovascular system and the increased respiration and its rate, respectively.