  The extreme variability event in the TeV emission of Mrk 421, recently
reported by the Whipple team, imposes the tightest limits on the typical size
of the TeV emitting regions in Active Galactic Nuclei (AGN). We examine the
consequences that this imposes on the bulk Lorentz factor of the emitting
plasma and on the radiation fields present in the central region of this Active
Nucleus. No strong evidence is found for extreme Lorentz factors. However,
energetics arguments suggest that any accretion in Mrk 421 has to take place at
small rates, compatible with an advection-dominated regime.
