  We calculate the time- and energy-dependent opacity of high-energy gamma-rays
attenuated by pair production interaction with accretion-flare photons that are
scattered by gas and dust surrounding thenuclei of active galaxies. We show
that the temporal behavior of the high-energy opacity cutoff can be used in
conjunction with the time history of the accretion flare to determine the
location of the gamma-ray emission site, as well as the column density and
scale height of the material surrounding the central engine. Reverberation
mapping using this technique is possible for nearby BL Lac objects and will be
particularly valuable when the 10 -- 100 GeV gamma-ray window is opened,
because there the attenuation of distant blazar radiation by the IIBR is
negligible.
