  We present echelle observations of the intrinsic UV absorption lines in the
Seyfert galaxy NGC 4151, which were obtained with the Space Telescope Imaging
Spectrograph (STIS) on the Hubble Space Telescope on 1999 July 19. The UV
continuum flux at 1450 Angstroms decreased by factor of about four over the
previous two years and there was a corresponding dramatic increase in the
column densities of the low-ionization absorption lines (e.g., Si II, Fe II,
and Al II), presumably as a result of a decrease in the ionizing continuum. In
addition to the absorption lines seen in previous low states, we identify a
large number of Fe II absorption lines that arise from metastable levels as
high as 4.1 eV above the ground state, indicating high densities (> 10^6
cm^-3). We find that the transient absorption feature in the blue wing of the
broad C IV emission, seen in a Goddard High Resolution Spectrograph spectrum
and thought to be a high-velocity C~IV component, is actually a Si II
fine-structure absorption line at a radial velocity of -560 km/s (relative to
systemic). We also demonstrate that the ``satellite'' emission lines of C~IV
found in International Ultraviolet Explorer spectra are actually regions of
unabsorbed continuum plus broad emission that become prominent when the UV
continuum of NGC 4151 is in a low state.
