  We discuss medium corrections of the nucleon-nucleon (NN) cross sections and
their influence on direct reactions at intermediate energies $\gtrsim 50$
MeV/nucleon. The results obtained with free NN cross sections are compared with
those obtained with a geometrical treatment of Pauli-blocking and with NN cross
sections obtained with Dirac-Bruecker methods. We show that medium corrections
may lead to sizable modifications for collisions at intermediate energies and
that they are more pronounced in reactions involving weakly bound nuclei.
