  We present the first measurement of the black hole (BH) mass function for
broad-line active galaxies in the local Universe. Using the ~9000 broad-line
active galaxies from the Fourth Data Release of the Sloan Digital Sky Survey,
we construct a broad-line luminosity function that agrees very well with the
local soft X-ray luminosity function. Using standard virial relations, we then
convert observed broad-line luminosities and widths into BH masses. A mass
function constructed in this way has the unique capability to probe the mass
region <10^6 M_sun, which, while insignificant in terms of total BH mass
density, nevertheless may place important constraints on the mass distribution
of seed BHs in the early Universe. The characteristic local active BH has a
mass of ~10^7 M_sun radiating at 10% of the Eddington rate. The active fraction
is a strong function of BH mass; at both higher and lower masses the active
mass function falls more steeply than one would infer from the distribution of
bulge luminosity. The deficit of local massive radiating BHs is a well-known
phenomenon, while we present the first robust measurement of a decline in the
space density of active BHs at low mass.
