  The largest group of sources identified by EGRET are Blazars. This sub-class
of AGN is well known to vary in flux in all energy bands on time-scales ranging
from a few minutes (in the optical and X-ray bands) up to decades (radio and
optical regimes). In addition to variations of the gamma-ray flux between
different viewing periods, the brightest of these sources showed a few
remarkable gamma-ray flares on time-scales of about one day, confirming the
extension of the ``Intraday-Variability (IDV)'' phenomenon into the GeV range.
We present first results of a systematic approach to study fast variability
with EGRET data. This statistical approach confirms the existence of IDV even
during epochs when no strong flares are detected. This provides additional
constraints on the site of the gamma-ray emission and allows cross-correlation
analyses with light curves obtained at other frequencies even during periods of
low flux. We also find that some stronger sources have fluxes systematically
above threshold even during quiescent states. Despite the low count rates this
allows explicit comparisons of flare amplitudes with other energy bands.
