The aim of this study was to investigate the relationship between heart rate variability (HRV) during exercise and ventilatory threshold (VT), using the MemCalc system which is superior in spectral analysis. Nine healthy male subjects with no cardiopulmonary disease performed an exercise test to exhaustion to determine VT on a bicycle ergometer. Low frequency (LF : 0.04-0.15 Hz), high frequency (HF : 0.15-0.40Hz) and LF/HF ratio power spectra were calculated by maximum entropy method (MEM) spectral analysis, using the MemCalc system. In each case, when the subject started exercise, the HF component declined rapidly during the first 30 seconds ; and compared to the resting value, it declined to approximately 5 % at VT. The possibility of using this phenomenon as a criterion for setting intensity of exercise is tinder consideration. The LF/HF ratio showed different patterns of variation among the subjects. A significant linear relationship was seen between the declining rate of the HF component and VT (r=0.93, p<0.001), suggesting a corres pondence between the heart's capability of adjusting to maintained exercise and high endurance capacity.