  Starting from the ~50000 quasars of the Sloan Digital Sky Survey for which
MgII line width and 3000 A monochromatic flux are available, we aim to study
the dependence of the mass of active black holes on redshift. We focus on the
observed distribution in the FWHM - nuclear luminosity plane, which can be
reproduced at all redshifts assuming a limiting BH mass, a maximum Eddington
ratio and a minimum luminosity (due to the survey flux limit). We study the
z-dependence of the best fit parameters of assumed distributions at increasing
redshift and find that the maximum mass of the quasar population evolves as
log[M_BH(max)/M_Sun]~0.3z+9, while the maximum Eddington ratio (~0.45) is
practically independent of cosmic time. These results are unaffected by the
Malmquist bias.
