  We consider the case of a scalar field, the inflaton, coupled to both lighter
scalars and fermions, and the study the relaxation of the inflaton via particle
production in both the linear and non-linear regimes. This has an immediate
application to the reheating problem in inflationary universe models. The
linear regime analysis offers a rationale for the standard approach to the
reheating problem, but we make a distinction between relaxation and
thermalization. We find that particle production when the inflaton starts in
the {\it non-linear} region is typically a far more efficient way of
transfering energy out of the inflaton zero mode and into the quanta of the
lighter scalar than single particle decay. For the non-linear regime we take
into account self-consistently the evolution of the expectation value of the
inflaton field coupled to the evolution of the quantum fluctuations. An
exhaustive numerical analysis reveals that the distribution of produced
particles is far from thermal, and the effect of open channels. In the
fermionic case, Pauli blocking begins to hinder the transfer of energy into the
fermion modes very early on in the evolution of the inflaton. We examine the
implications of our results to the question of how to calculate the reheating
temperature of the universe after inflation.
