  We extend our analysis for scalar fields in a Robertson-Walker metric to the
electromagnetic field and Dirac fields by the method of invariants. The issue
of the relation between conformal properties and particle production is
re-examined and it is verified that the electromagnetic and massless spinor
actions are conformal invariant, while the massless conformally coupled scalar
field is not. For the scalar field case it is pointed out that the violation of
conformal simmetry due to surface terms, although ininfluential for the
equation of motion, does lead to effects in the quantized theory.
