  We present an extension of the QCD sum rule method in the external fields so
as to determine the induced pseudoscalar coupling constant g_P, which tests the
validity of the partially conserved axial current (PCAC) hypothesis. This is
essentially that we pick out the "higher-order" effects of both the hadron and
quark (QCD) sides. A specific QCD sum rules for g_P is obtained and its
prediction is briefly analyzed. It turns out that the final prediction on g_P
is extremely stable. In view of the versatile nature of the present QCD sum
rule methods, we appendix some discussions on the possible future of the
method.
