  We investigate the screening effects on the nucleon-nucleon elastic cross
section inside nuclear matter at zero and finite temperature. The N-N
interaction is described phenomenologically via meson exchanges ($\sigma$,
$\pi$ and $\omega$) in the framework of a relativistic lagrangian model. The
expressions for the in-medium meson propagators, which take into account the
effects of matter polarization and renormalized vacuum polarization, are used
in the calculation of the various meson contributions to the total cross
section in the one-boson exchange approximation. The final expressions allow a
satisfactory fit of the Coulomb substracted proton-proton cross section data in
vacuum. At finite density or temperature the cross section is reduced as
compared to the vacuum values and compatible with the existing experimental
constraints. At high density the $p$-$p$ differential cross section gets
increasingly forward and backward peaked. The $\sigma$-$\omega$ mixing plays a
key role in the understanding of the results.
