  We extend, for the case of a general scalar potential, the inflaton-graviton
Feynman rules recently developed by Iliopoulos {\it et al.} As an application
we compute the leading term, for late co-moving times, of the one loop back
reaction on the expansion rate for $V(\phi) = \frac12 m^2 \phi^2$. This is
expressed as the logarithmic time derivative of the scale factor in the
coordinate system for which the expectation value of the metric has the form:
$<0 | g_{\mu\nu}({\bar t},{\vec x}) | 0 > dx^{\mu} dx^{\nu} = - d{\bar t}^2 +
a^2({\bar t}) d{\vec x} \cdot d{\vec x}$. This quantity should be a gauge
independent observable. Our result for it agrees exactly with that inferred
from the effect previously computed by Mukhanov {\it et al.} using canonical
quantization. It is significant that the two calculations were made with
completely different schemes for fixing the gauge, and that our computation was
done using the standard formalism of covariant quantization. This should settle
some of the issues recently raised by Unruh.
