In recent years, the assessment of body composition (BC) has been reframed by the hormonal effect that skeletal muscle and adipose tissue have in the body, including the autonomic nervous system (ANS). PURPOSE: To determine the relationship between the BC and ANS response in a population of individuals with different levels of physical activity. METHODS: 63 individuals (31 men 19.9 ± 2.4 years and 19.6 ± 32 women 2.0 years) were evaluated on their body composition (bioimpedance and anthropometry) and the ANS activity at rest through the Heart Rate Variability. RESULTS: In men a directly proportional relationship between the percentage of muscle mass (evaluated for both methods) and indicators of parasympathetic activity was found: The Root Mean Square Successive Differences RMSSD (r=0.40; p<0.05), the deviation of the scatterogram plot in the “short” direction SD1 (r=0.40; p<0.05), and Vagal Cardiac index VCI (r=0.46, p<0.05), while body fat was associated with sympathetic indicators as the ratio Low Frequency / High Frequency (r=0.36; p<0.05) and sympathetic cardiac index SCI (r=0.36, p<0.05), the SCI was inversely proportional to muscle mass too (r=-0.41 p<0.05). In women only heart rate was directly related to adipose tissue (r=0.35; p<0.05) and inversely with muscle mass (r=-0.42; p<0.05). CONCLUSIONS: The muscle mass has an influence on the ANS by increasing parasympathetic activity and reducing the sympathetic activity, the evidence suggests that there are some hormones produced by the muscles (myokines) like the Fibroblast Growth Factor 21 or the Brain Derivate neurotrophic Factor that produce this effect. For the other hand, some adipokines like the Tumoral Necrosis Factor alpha and the leptin contribute to have greater sympathetic and lower parasympathetic activity.Table: No title available.