  A systematic study of the $\pi^-/\pi^+$ ratio in heavy-ion collisions with
the same neutron/proton ratio but different masses can help single out effects
of the nuclear mean field on pion production. Based on simulations using the
IBUU04 transport model, it is found that the $\pi^-$ / $\pi^+$ ratio in head-on
collisions of $^{48}$Ca+$^{48}$Ca, $^{124}$Sn +$^{124}$Sn and
$^{197}$Au+$^{197}$Au at beam energies from 0.25 to 0.6 \agev increases with
increasing the system size or decreasing the beam energies. A comprehensive
analysis of the dynamical isospin fractionation and the \rpi ratio as well as
their time evolution and spatial distributions demonstrates clearly that the
\rpi ratio is an effective probe of the high-density behavior of the nuclear
symmetry energy.
