  The response of mesoscopic superconductors to an ac magnetic field is
investigated both experimentally and with numerical simulations. We study small
square samples with dimensions of the order of the penetration depth. We obtain
the ac susceptibitity $\chi=\chi'+i\chi''$ at microwave frequencies as a
function of the dc magnetic field $H_{dc}$. We find that the dissipation, given
by $\chi''$, has a non monotonous behavior in mesoscopic samples. In the
numerical simulations we obtain that the dissipation increases before the
penetration of vortices and then it decreases abruptly after vortices have
entered the sample. This is verified experimentally, where we find that
$\chi''$ has strong oscillations as a function of $H_{dc}$ in small squares of
Pb.
