  The $q^2$ dependence of the $\pi$-$\eta$ mixing amplitude is examined with
the use of QCD sum rules. The linear slope of the mixing function $\theta(q^2)$
is found to be much smaller than that for $\rho$-$\omega$ mixing. Thus the
mixing amplitude is approximately the same in the space-like region as in the
time-like one, and one may neglect the $q^2$ dependence of the mixing. A
comparison between a hadron-meson, an effective chiral model, and the QCD sum
rules approaches is made.
