  We investigate the modification induced on the nucleon-nucleon cross section
by screening of the interaction inside nuclear matter. The interaction is
described by the relativistic one boson exchange of $\sigma$, $\omega$ and
$\pi$ mesons. Medium effects enter as loop corrections to the meson propagators
and are characterized not only by density, but also by temperature and velocity
with respect to the center of mass of the collision. The cross section displays
peaks for some specific values of the velocity, corresponding to the excitation
of zero-sound modes of the longitudinal $\omega$ field. The enhancement factor
amounts to about 1.5 under reasonable conditions. It increases with density and
is reduced at finite temperature. These findings may have verifiable
consequences on the angular dependence of the measurable secondary particle
distributions.
