  The in-medium modification of nucleon-nucleon (NN) cross sections is
investigated by means of particle production in heavy ion collisions (HIC) at
intermediate energies. In particular, the density dependence of the {\it
inelastic} cross sections considerably affects the pion and kaon yields and
their rapidity distributions. However, the $(\pi^{-}/\pi^{+})$- and
$(K^{+}/K^{0})$-ratios depend only moderately on the in-medium behavior of the
inelastic cross sections. It turns out that particle ratios seem to be robust
observables in determining the nuclear equation of state (EoS) and,
particularly, its isovector sector.
