  We present the first observations of a Seyfert galaxy with the echelle
gratings on the Space Telescope Imaging Spectrograph (STIS), which provide
high-resolution (R = 40,000) coverage of the intrinsic UV absorption lines in
NGC 5548. We confirm the presence of five kinematic components of absorption in
Lyman-alpha, C IV, and N V at radial velocities of -160 to -1060 km/s with
respect to the emission lines, and find an additional Lyman-alpha$ component
near the systemic velocity, which probably arises in the interstellar medium of
the host galaxy. Compared to GHRS spectra of the N V and C IV absorption
obtained ~2 years earlier, the kinematic components have not changed in radial
velocity, but the ionic column densities for two components have decreased. We
attribute these variations to changes in the total column of gas, but for one
component, we cannot rule out changes in the ionization of the gas.
  We have calculated photoionization models to match the UV column densities
from each of the five components associated with the nucleus. In four of the
components, the ionization parameters (U = 0.15 - 0.80) and effective hydrogen
column densities (N = 6.0 x 10^18 cm^-2 -- 2.8 x 10^20 cm^-2) cannot produce
the O VII and O VIII absorption edges seen in the X-ray warm absorber. The
remaining component is more highly ionized (U = 2.4, N = 6.5 x 10^21 cm^-2) and
our model matches the previously observed X-ray absorption columns. This
component is therefore likely to be responsible for the X-ray warm absorber. It
also has the highest outflow velocity and showed the largest variations in
column density.
