  We have analyzed two long BeppoSAX observations of the bright Seyfert galaxy
NGC 4151, searching for short timescale (10-200 ksec) X-ray spectral
variability. The light curve of a softness ratio, chosen as most sensitive to
pinpoint changes of the column density of the absorbing gas along the line of
sight, shows significant variations. We try to model these variations by
performing a detailed, time resolved, spectral analysis. We find significant,
large (factors of 1.5-6) variations of the absorber column densities on time
scales of 40-200 ksec. These values are 10-100 times shorter than those found
by Risaliti et al. 2002 in a sample of Seyfert 2 galaxies, and provide strong
constraints on the geometry of the obscuring medium.
