  During cosmological inflation, it has been suggested that fields coupled to
the inflaton can be excited by the slow-rolling inflaton into a quasi-stable
non-vacuum state. Within this scenario of ``warm inflation'', this could allow
for a smooth transition to a radiation dominated Universe without a separate
reheating stage and a modification of the slow roll evolution, as the heat-bath
backreacts on the inflaton through friction. In order to study this from first
principles, we investigate the dynamics of a scalar field coupled to the
inflaton and N light scalar boson fields, using the 2PI-1/N expansion for
nonequilibrium quantum fields. As a first step we restrict ourselves to
Minkowski spacetime, interpret the inflaton as a time-dependent background, and
use vacuum initial conditions. We find that the dominant effect is particle
creation at late stages of the evolution due to the effective time-dependent
mass. The further transfer of energy to the light degrees of freedom and
subsequent equilibration only occurs after the end of inflation. As a
consequence, the adiabatic constraint, which is assumed in most studies of warm
inflation, is not satisfied when starting from an initial vacuum state.
