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