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The rapidly variable Seyfert~1 galaxy NGC~4151 will be the subject of an intensive monitoring campaign with IUE, Rosat, GRO, ASCA and ground-based telescopes in December 1993. We will report preliminary results of 9.3 days of continuous monitoring with IUE, at a sampling rate of once every $\sim$80 min, an order of magnitude faster than that achieved in previous AGN monitoring campaigns. Observations, data reduction and analysis will be discussed, focusing especially on accurate measurement of continuum and line fluxes in $\sim$400 spectra. We will report on the preliminary search for a correlation between variations in the 1400~\AA\ and 2800~\AA\ continuum bands. This will provide a critical test of any model which claims that the ultraviolet is powered by an accretion disk, since such models predict that the 1400~\AA\ variations should lead those at 2800~\AA\ by at least 1--2~hr. We will also plan to measure in unprecedented detail the temporal cross-correlation function of the highest ionization and most rapidly responding emission lines. This should strongly constrain the distribution of the innermost broad line gas.
