  It is shown that a 25(20)% difference between the decay constants
$f_{D_s}(f_{B_s})$ and $f_D(f_B)$ occurs due to large differences in the pole
masses of the $s$ and $d(u)$ quarks. The values $\eta_D =f_{D_s}/f_D\approx
1.23(15)$, recently observed in the CLEO experiment, and
$\eta_B=f_{B_s}/f_B\approx 1.20$, obtained in unquenched lattice QCD, can be
reached only if the running mass $m_s$ at low scale is $m_s(\sim 0.5 $ GeV)$=
170 - 200$ MeV. Our results follow from the analytical expression for the
pseudoscalar decay constant $f_{\rm P}$ based on the path-integral
representation of the meson Green's function.
