  We discuss the physical effects causing a modification of resonance masses,
widths and even shapes in a dilute hadronic gas at late stages of heavy ion
collisions. We quantify the conditions at which resonances are produced at
RHIC, and found that it happens at $T\approx 120 MeV$. Although in the pp case
the ``kinematic'' effects like thermal weighting of the states is sufficient,
in AA we see a clear effect of dynamical interaction with matter, both due to a
variety of s-channel resonances and due to t-channel scalar exchanges. The
particular quantity we focus mostly on is the $\rho$ meson mass, for which
these dynamical effects lead to about -50 MeV shift, on top of about -20 MeV of
a thermal effect: both agree well with preliminary data from STAR experiment at
RHIC. We also predict a complete change of shape of $f_0(600)$ resonance, even
by thermal effects alone.
