  We present the results of an intensive ultraviolet monitoring campaign on the
Seyfert 1 galaxy NGC 4151, as part of an effort to study its short time-scale
variability over a broad range in wavelength. The nucleus of NGC 4151 was
observed continuously with the {\it International Ultraviolet Explorer} (IUE)
for 9.3 days, yielding a pair of LWP and SWP spectra every $\sim$70 minutes,
and during four-hour periods for 4 days prior to and 5 days after the
continuous monitoring period. The sampling frequency of the observations is an
order of magnitude higher than that of any previous UV monitoring campaign on a
Seyfert galaxy.
  The continuum fluxes in bands from 1275 \AA\ to 2688 \AA\ went through four
significant and well-defined ``events'' of duration 2 -- 3 days during the
continuous monitoring period. We find that the amplitudes of the continuum
variations decrease with increasing wavelength, which extends a general trend
for this and other Seyfert galaxies to smaller time scales (i.e., a few days).
The continuum variations in all of the UV bands are {\it simultaneous} to
within an accuracy of about 0.15 days, providing a strict constraint on
continuum models. The emission-line light curves show only one major event
during the continuous monitoring (a slow rise followed by a shallow dip), and
do not correlate well with continuum light curves over the (short) duration of
the campaign, because the time scale for continuum variations is apparently
smaller than the response times of the emission lines.
