  It is shown that there exist symmetry constraints for non--leptonic weak
amplitudes which emerge when the $1/N_c$--expansion restricted to the leading
and next--to--leading approximations only is systematically combined with
$\chi$PT limited to the lowest non--trivial order. We discuss these constraints
for the couplings $\mbox{\bf g}_8$ and $\mbox{\bf g}_{27}$ of $\Delta S=1$
transitions and the $\mbox{\bf B}_{K}$--parameter of $K^0-{\bar K}^0$ mixing.
