We present emission-line and ultraviolet continuum observations of a type I Seyfert galaxy in which the time resolution is adequate for describing the character of variability. Using the IUE satellite, the nucleus of NGC 5548 was observed every 4 days for a period of 8 months. Its mean properties - continuum shape, line ratios - are not unusual for type I Seyfert galaxies, but it was found to be strongly variable. The ultraviolet continuum flux and broad emission line fluxes varied significantly, going through three large maxima and three deep minima. The ratio of maximum to minimum flux was ≃4.5 for the continuum at 1350 Å, and the continuum was significantly bluer when it was brighter. The high-ionization emission lines showed the strongest variations, with N v λ1240 and He II λ1640 exhibiting maximum-to-minimum flux ratios as high as those of the continuum. Intermediate-ionization lines, including Lyoα λ1216, C IV 1549, and C III] λ1909, had maximum-to-minimum amplitudes of ∼2, and Mg II λ2798, the lowest ionization line, exhibited the smallest amplitude fluctuations, ∼ 1.3. The great majority of all variations were well resolved in time. Apart from Mg II λ2798, the emission-line variations correlate extremely well with those of the 1350 Å continuum if allowance is made for a systematic delay, lending qualitative support to the view that photoionization by the nuclear continuum is responsible for driving the emission lines. The delay of a given line seems to depend on the degree of ionization of its species. The He II λ1640 and N V λ1240 features exhibit the shortest delay, Δt ∼ 4-10 days, while the Lyα λ1216 and C IV λ1549 lines yield 8-16 days. The Si IV + O IV] λ1402 feature and the C III] λ1909 line exhibit significantly larger delays, between 12 and 34 days. In the case of Mg II λ2798, the cross-correlation is broad and shallow, so that the delay is only loosely constrained, Δt ∼ 34-72 days.
