This study examined the influence of body mass index (BMI) on heart rate variability (HRV) at rest and examination stress. 123 healthy female students with a mean age of 20.66±0.12 years volunteered for the experiment: 12 thin subjects (TS) (BMI<18.5); 9 pre-obese subjects (PO) (BMI = 25 – 29,99); 102 nonobese controls (NC) (BMI = 18.5 – 24.99). In all participants, we performed 5-minute electrocardiogram recording at rest and shortly before real life stress. Main outcome measures included heart rate (HR) and HRV parameters. Kruskal-Wallis ANOVA was used to assess significance of associations. Baseline characteristics in NC group did not differ significantly from the TS and PO groups. During stress, TS had significantly elevated HR and decreased SDNN (94.18±3.95 and 0.036±0.005, respectively) when compared with NC (82.93±1.13 and 0.047±0.002, respectively) and PO (73.33±3.12 and 0.063±0.007, respectively). In comparison with the two other groups, TS showed a significant decrease TF (TS= 1797,55± 367,53; PO=3551,76 ±232,81; NC= 6386.67±1092.11), and a significant decrease of the power of the LF (TS=570.65±116.44; P0= 1242,45 ±104.15; NC= 2216.11±454.61). There were no differences between the groups in HF and LF/HF. The results are consistent with the hypothesis that low BMI is characterized by a tendency towards heightened stress-induced physiological activation. Keywords: heart rate variability, body mass index Supported by the Ministry of Education and Science of Russia (grant N 4.4904.2011) and RGNF (grant N 12-16-21014).