We investigated the change in activity of the sympathetic nervous system (SNS) in high-sucrose diet (HSD)-induced obese rats compared with controls. Power spectral analyses of R-R interval variability were performed to obtain the low frequency (LF, 0.04-0.699 Hz) and high frequency (HF, 0.7-3.0 Hz) powers. The percents of fat mass to body weight (%F/BW) and fat to muscle ratios (F/M) were significantly increased in HSD-fed rats. Plasma glucose, leptin, and triglyceride concentrations in rats fed with HSD were significantly increased. LF in normalized units (LFn), which represents both sympathetic and parasympathetic activities, was significantly increased whereas HF in normalized unit (HFn), which represents parasympathetic activity, was significantly decreased in HSD-fed rats. LF/HF, which represents sympathetic activity, was significantly increased in HSD-fed rats and was correlated with leptin (r=0.549, p<0.023), %F/BW (r=0.513, p<0.035), F/M (r=0.536, p<0.038), and triglyceride (r=0.497, p<0.042). When adjusted for leptin concentrations, however, LF/HF of HSD-fed rats was significantly decreased. In conclusion, HSD-induced obese rats showed increased LF/HF, which was significantly decreased by adjustment for leptin concentrations. We suggest that stimulating effect of leptin on SNS is reduced, which might play a role in induction of obesity by HSD.