The present study was designed to investigate the effect of capsaicin, a pun-gent component of red pepper, on the sympathetic nervous system (SNS) activity and en-ergy metabolism in 16 age- and height-matched lean and obese young women. The sympa-tho-vagal activities were assessed by means of spectral analysis of heart rate variability (HRV) during the resting condition and after the meal (2, 016 kJ) with capsaicin (3mg). Energy expenditure was also measured under the two conditions. There was no significant difference in any of the parameters of the HRV between the obese and control groups at rest. After the capsaicin diet, however, the very low frequency component (0.007-0.035 Hz) as-sociated with thermogenesis (315.8±78.0 vs. 814.8±211.7 ms²⋅Hz-1, p<0.05) as well as its responsiveness (delta changes: 14.6±104.4 vs. 369.2±121.7 ms²⋅Hz-1, p<0.05) were significantly lower in the obese than the control group. Energy expenditure was signifi-cantly increased in the control group after the meal (5, 574.7±221.2 to 6, 114.7±239.0kJ day-1; p<0.01), but no such significant thermogenic response was detected in the obese group despite nearly identical lean body mass of the control group. Our data indicate that regardless of the resting level of sympatho-vagal activities, the reduced sympathetic respon-siveness to physiological perturbation such as a capsaicin diet, which may cause impaired diet-induced thermogenesis and further weight gain, could be an important etiological fac-torleading to obesity in young women.