  In this paper it is shown how the generating functional for Green's functions
in relativistic quantum field theory and in thermal field theory can be
evaluated in terms of a standard quantum mechanical path integral. With this
calculational approach one avoids the loop-momentum integrals usually
encountered in Feynman perturbation theory, although with thermal Green's
functions, a discrete sum (over the winding numbers of paths with respect to
the circular imaginary time) must be computed. The high-temperature expansion
of this sum can be performed for all Green's functions at the same time, and is
particularly simple for the static case. The procedure is illustrated by
evaluating the two-point function to one-loop order in a $\phi^3_6$ model.
