  We describe a simple and efficient mechanism by which very heavy particles
are copiously created from a primordial inflationary epoch. It works for scalar
fields which are massless or very light during inflation and acquire a large
mass right after the end of inflation. Such particles can exist in realistic
scenarios, as we illustrate with several examples in the context of both field
and string theory. Long-wavelength fluctuations of these fields are generated
during inflation with an almost scale-invariant spectrum and may give the
dominant contribution to the energy density of the heavy fields at late times.
Applications of our results to superheavy dark matter and leptogenesis are
discussed.
