  This paper sets out to measure the timescale of quasar variability with a
view to new understanding of the size of accretion discs in active galactic
nuclei. Previous attempts to measure such timescales have been based on
sparsely sampled data covering small ranges of time. Here we combine data from
two large scale monitoring programmes to obtain Fourier power spectra of light
curves covering nearly three orders of magnitude in frequency in blue and red
passbands. If the variations are interpreted as due to gravitational
microlensing, then timescale measurements in the observer's frame imply a
minimum mass for the microlensing bodies of around $0.4 M_{\odot}$. On the
assumption that the variations are intrinsic to the quasars, a correction must
be made for time dilation. In this case the power spectrum shows a break
corresponding to a timescale of about 11 years. This timescale is used to
measure the size of the accretion disc, which is found to be about $10^{-2}$ pc
or 10 light days, in agreement with limits set by self-gravitation and
coincident with the broad line region of the active galactic nucleus. It is
suggested that the broad line region may be associated with the break up of the
outer part of the accretion disc.
