Variable Seyfert nuclei exhibit correlations between emission-line and continuum luminosity consistent with photoionization, although the emission lines (including recombination features such as Lyα) tend to respond nonlinearly to changes in the continuum. A nonlinear response will result if some of the broad-line region (BLR) clouds become fully ionized in hydrogen, such that their emission measure does not then vary in proportion to the incident continuum flux. For thin clouds, emission in transitions from heavy elements may grow or decline with increasing incident flux, reflecting changes in the ionization structure of the cloud. Direct evidence for a fully ionized component within the BLR is provided by correlation analysis of ultraviolet line response in NGC 5548 (Sparke, 1993).