Can super-Hubble metric perturbations be amplified exponentially during pre
heating? Yes. An analytical existence proof is provided by exploiting the c
onformal properties of massless inflationary models. The traditional conser
ved quantity zeta is nonconserved in many regions of parameter space. We in
clude back reaction through the homogeneous parts of the inflaton and prehe
ating fields and discuss the role of initial conditions on the post-preheat
ing power spectrum. Maximum field variances are strongly underestimated if
metric perturbations are ignored. We illustrate this in the case of strong
self-interaction of the decay products. Without metric perturbations, prehe
ating in this case is very inefficient. However, metric perturbations incre
ase the maximum field variances and give alternative channels for the reson
ance to proceed. This implies that metric perturbations can have a large im
pact on calculations of relic abundances of particles produced during prehe
ating.