  I estimate the degree of local chemical equilibration for $\pi$, $\rho$ and
$\omega$ mesons in ultra--relativistic nuclear collisions. The $\pi$ and $\rho$
mesons remain near chemical equilibrium in all cases, while the $\omega$ meson
density is typically 30--50\% higher than the equilibrium value for O+Ag
collisions at $\sqrt{s}=20$ GeV. If chemical reactions are turned off, the
$\omega$ meson density is much larger, 2.5--3 times its equilibrium value.
Thus, $\omega$ mesons may provide the most sensitive tests for the degree of
chemical equilibration in nuclear collisions.

Nuclear Theory