  A method to calculate the mean squared matrix element of weak interaction
between compound states is developed. The result is expressed in terms of
matrix elements of the nucleon-nucleon strong and weak interactions times the
Fermi distribution functions at finite temperature. Numerical calculations for
$^{233}$Th are in excellent agreement with recent measurements of parity
nonconservation effects in neutron capture. In fact, our calculations prove
that the factor of dynamical enhancement (ratio of compound-nucleus effect to
single-particle one) really exceeds 100, thus making it unnesessary to assume a
value of the weak constant bigger than standard one ($g\simeq 10^{8} \epsilon
\sim 1\div 4$).
