We use a simple renormalization group argument to demonstrate that
the large top mass limit of the axial neutral current in QCD is
unambiguously determined by the renormalization of the singlet axial
current and suggests a natural scheme independent normalization
condition for the latter. By calculating the anomalous dimension of
the singlet axial current at three-loop level in the $\msbar$ scheme
we derive the result for the top quark decoupling from the axial
neutral current valid up to (and including) the terms of order
$\as^2$.