  A semiclassical calculation of particle production by a scalar field in a
potential is performed. We focus on the particular case of production of
fermions by a Nambu-Goldstone boson $\theta$. We have derived a (non)local
equation of motion for the $\theta$-field with the backreaction of the produced
particles taken into account. The equation is solved in some special cases,
namely for purely Nambu-Goldstone bosons and for the tilted potential $
U(\theta ) \propto m^2 \theta^2 $. Enhanced production of bosons due to
parametric resonance is investigated; we argue that the resonance probably
disappears when the expansion of the universe is included. Application of our
work on particle production to reheating and an idea for baryogenesis in
inflation are mentioned.
