  We have evaluated the Bloch-Siegert shift for the different values of
magnetic field's strengths defined at astrophysical conditions, i.e. when the
stars with the strong surface magnetic fields are taken as a powerful pumping
source of radiation. It is found that the additional shift of resonant
frequency should be taken into account in the search for the time variation of
the fundamental constants. The main conclusion is that the influence of the
electromagnetic field shift should be considered carefully in each special case
of the corresponding frequency determination.
