  The nucleon mass corrections are calculated to all polarized structure
functions for neutral and charged current deep inelastic scattering in lowest
order in the coupling constant. The impact of the target mass corrections on
the general relations between the twist--2 and twist--3 parts of the structure
functions is studied and three new relations between the twist--3 contributions
are derived. The size of nucleon mass corrections for the $g_1$ and $g_2$
structure functions are estimated.
