  We have analyzed the two longest (elapsed time > 3 days) BeppoSAX
observations of the X-ray brightest Seyfert galaxy, NGC 4151, to search for
spectral variability on time-scales from a few tens of ksec to years. We found
in both cases highly significant spectral variability below ~ 6 keV down to the
shortest time-scales investigated. These variations can be naturally explained
in terms of variations in the low energy cut-off due to obscuring matter along
the line of sight. If the cut-off is modeled by two neutral absorption
components, one fully covering the source and the second covering only a
fraction of the source, the shortest time-scale of variability of a few days
constrains the location of the obscuring matter to within 3.4 X 10^4
Schwarzschild radii from the central X-ray source. This is consistent with the
distance of the Broad Emission Line Region, as inferred from reverberation
mapping, and difficult to reconcile with the parsec scale dusty molecular torus
of Krolik & Begelman (1988). We have also explored a more complex absorption
structure, namely the presence of an ionized absorber. Although the behaviour
of the ionization parameter is nicely consistent with the expectations, the
results are not completely satisfactory from the statistical point of view.
  The overall absorption during the 2001 December observation is lower than in
all other historical observations with similar 2-10 keV flux. This suggests
that absorption variability plays a crucial role in the observed flux
variability of this source.
