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We present the first results of a combined HST /{\em IUE}/ground-based spectroscopic monitoring campaign on the Seyfert~1 galaxy NGC~5548 that was undertaken in order to address questions that require both higher temporal resolution and higher signal-to-noise ratios than were obtained in our previous multiwavelength monitoring of this galaxy in 1988 -- 89. IUE spectra were obtained once every two days for a period of 74 days beginning on 1993 March 14. During the last 39 days of this campaign, spectroscopic observations were also made with the HST Faint Object Spectrograph (FOS) on a daily basis. Ground-based observations, consisting of 165 optical spectra and 77 photometric observations, were made between 1992 October and 1993 September. We find (1) any lag present between the 1350\,\AA\ continuum and the 5100\,\AA\ continuum must amount to less than about one day, (2) the variations in the highest ionization lines observed, He\,{\sc ii} \,$\lambda1640$ and N\,{\sc v} \,$\lambda1240$, lag behind the continuum variations by somewhat less than 2 days, (3) the velocity field of the C\,{\sc iv}-emitting region is not dominated by radial motion, (4) the H$\beta$ line flux variations follow those of the continuum with a time lag of around two weeks, (5) the UV/optical continuum becomes ``harder'' as it gets brighter and the shorter wavelengths show characteristics of shorter timescale variability, (6) the highest ionization emission lines have the shortest lags, thus indicating radial ionization stratification of a broad-line region that spans over an order of magnitude range in radius.
We are grateful for support of this program by NASA through grants GO-3484.01-91A (through STScI) and NAG5-1824 and by NSF through various grants to the ground-based observers.
