  Using a relativistic transport model for heavy ion collisions at energies
that are below the threshold for kaon and antikaon production in
nucleon-nucleon collisions, we study how their abundances approach the
canonical equilibrium during the collisions. We find that kaons are far from
chemical equilibrium at the initial and high density stage, and they approach
equilibrium only during the expansion stage of the collisions when their
production rate is small and becomes comparable to their annihilation rate. In
contrast, antikaons approach chemical equilibrium much earlier but eventually
fall out of equilibrium again as a result of their large annihilation cross
sections in nuclear matter.
