Autonomic nervous system activity is involved in body weight regulation. We assessed whether the body mass index (BMI) is related to the autonomic nervous system activity as assessed by heart rate variability (HRV). Twenty-five adult normotensive, euglycemic healthy males (M) and females (F) were studied (M/F=13/12). BMI was assessed in each individual. HRV was assessed and the domains of low frequencies (LF, index of the sympathetic modulation) and high frequencies (HF, index of the parasympathetic modulation) were measured. Data were statistically analyzed and are presented as mean+/-s.d. Mean BMI did not correlate with either HF or LF. It inversely related to HF (r=-0.50, P<0.01), whereas its relationship with LF was marginally significant (r=-0.39, P=0.05). The HF in individuals with BMI <20 kg/m(2) was significantly higher from those measured in the remaining subjects (P<0.05). The results support the role of parasympathetic activity in influencing BMI through likely modulation of body weight.