  Thermodynamical variables and their time evolution are studied for central
relativistic heavy ion collisions from 10.7 to 160 AGeV in the microscopic
Ultrarelativistic Quantum Molecular Dynamics model (UrQMD). The UrQMD model
exhibits drastic deviations from equilibrium during the early high density
phase of the collision. Local thermal and chemical equilibration of the
hadronic matter seems to be established only at later stages of the quasi-
isentropic expansion in the central reaction cell with volume 125 fm$^{3}$.
distributions at all collision energies for $t\geq 10 fm/c$ with a unique
Baryon energy spectra in this cell are approximately reproduced by Boltzmann
rapidly dropping temperature. At these times the equation of state has a simple
form: $P \cong (0.12-0.15) \epsilon$. At 160 AGeV the strong deviation from
chemical equilibrium is found for mesons, especially for pions, even at the
late stage of the reaction. The final enhancement of pions is supported by
experimental data.
