  We develop a systematic chiral perturbation expansion for the calculation of
meson-exchange currents in nuclei and apply the formalism to nuclear axial
currents. We summarize the principal results of such a calculation to one loop
order on nuclear axial-charge transitions which provides a strong support to
the conjecture of ``chiral filter phenomenon" in nuclear medium. The use of
heavy baryon chiral perturbation theory enables us to obtain a remarkably
simple result valid next to the leading order in chiral counting. The dominant
role of a soft-pion exchange in axial-charge transitions in heavy nuclei is
confirmed. An important, {\it albeit} indirect, consequence of our result on
the empirically observed enhancement in axial-charge transitions in heavy
nuclei is pointed out.
