The purpose of the present study was to develop a new method that enables individualized determination of the optimal exercise intensity for health promotion. Our study was based on the following observations : (1) physical activity at ventilatory threshold (VT) has been useful for enhancing physical fitness and even improving medical conditions such as heart failure, hypertension and diabetes, (2) exercise intensity at VT is characterized by suppressed vagal activity, and (3) vagal activity can be evaluated by analyzing heart rate variability (HRV) . In the first study we defined a criteria for determining the exercise intensity corresponding to VT using HRV analysis (heart rate variability threshold, THRV) . In 16 normal subjects, a time series of ECG RR interval were recorded and the means of the sum of the squared differences in successive RR intervals (MSSDs) were calcu-lated during a ramp exercise test with a cycle ergometer. Based on the values of MSSD and the dif-ferences in successive MSSDs (ΔMSSD) at the intensity of VT, we defined the criteria of THRV as follows : MSSD<25 msec²and ΔMSSD<6 msec². Another exercise test with a cycle ergo-meter was performed to evaluate the relationship between THRV and VT in 63 normal subjects. Heart rate (HR) and oxygen uptake (VO₂/wt) at THRV were 111.8±13.2 beats/min and 15.2±4.4 ml/kg/min, and HR and VO₂/wt at VT were 116.2±11.6beats/min and 16.5±3.7ml/kg/min, respectively. There was a significant correlation between THRV and VT (HR : r=0.82, p<0.001, VO₂/wt : r=0.88, p< 0.001) . Thus, THRV and VT provided almost identical exercise intensities. As a result, we propose that, similar to VT, THRV can be used as an indicator of the optimal exercise intensity suitable for health promotion in normal subjects.