b-AR has a pivotal role in the functions of the cardiac autonomic nervous system. Activation of b 1 -AR provides strong stimulus to increase the frequency and contractility of the heart, whereas the activation of b 2 -AR results in smooth muscle relaxation and increased cardiac output with less extent compared to b 1 -AR. Thus, the connection between b-AR and HRV is plausible and warrants evaluations. Limited evidences suggest that variations in genes coding subtypes of b-AR may be associated with heart rate or HRV. For example, Ser49Gly polymorphism in b 1 -AR gene has been found to be associated with resting heart rate [14, 15] , and an association of b 2 -AR gene polymorphisms with spectral components of HRV measures has been reported in a relatively small healthy adult male sample [11] .