  This paper describes perturbative framework, on the basis of the
closed-time-path formalism, in terms of quasiparticle picture for studying
quasiuniform relativistic quantum field systems near equilibrium and
nonequilibrium quasistationary systems. Two calculational schemes are
introduced, the one is formulated on the basis of the initial-particle
distribution function and the one is formulated on the basis of the
``physical''-particle distribution function. It is shown that both schemes are
equivalent and lead to a generalized kinetic or Boltzmann equation. Concrete
procedure of computing a generic amplitude is presented.
