Emission-line variability data on NGC 5548 argue strongly for the existence of a mass of order 7 × 10 7 M ☉ within the inner few light-days of the nucleus in the Seyfert 1 galaxy NGC 5548. The time-delayed response of the emission lines to continuum variations is used to infer the size of the line-emitting region, and these determinations are combined with measurements of the Doppler widths of the variable line components to estimate a virial mass. The data for several different emission lines spanning an order of magnitude in distance from the central source show the expected V ∝ r -1/2 correlation and are consistent with a single value for the mass.
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