  We describe the creation and evolution of electron-positron pairs in a strong
electric field as well as the pairs annihilation into photons. The formalism is
based on generalized Vlasov equations, which are numerically integrated. We
recover previous results about the oscillations of the charges, discuss the
electric field screening and the relaxation of the system to a thermal
equilibrium configuration. The timescale of the thermalization is estimated to
be $\sim 10^{3}-10^{4} \hbar /m_{e}c^{2}$.
