  We set up a formalism for calculating the energy density generated in a
quantized massive scalar field in the course of the drastic change in spacetime
geometry at the end of the inflationary era. The calculation relies on the
notion of adiabatic vacuum. The Bogolubov coefficients are computed by
employing the sudden approximation. After obtaining a general formula, we
calculate explicitly the energy density generated in a particle species with
$m/H \ll1$, where $m$ is the particle mass and $H$ is the Hubble constant
during the inflationary epoch. We find the contribution of the long-wavelength
modes to be $\propto H^5/m$. If such particles are very weakly interacting,
they can come to dominate the total energy density in the Universe. Other
cosmological implications are also discussed.
