  We consider the interaction between perturbations in the inflaton and in the
metric during the preheating phase in simple inflationary models. By
numerically integrating the Einstein field equations we are able to gauge the
impact of non-linear gravitational effects on preheating for the first time. In
the lambda phi^4 model we find a large increase in the amplitude of sub-Hubble
metric modes, beyond that due to gravitational collapse alone. There is
significant mode-mode coupling and the amplification is eventually terminated
by back reaction effects. We suggest that such enhancement of inhomogeneity
will change the behaviour of the post-inflationary universe.
