  The velocity dispersion of nearby stars in the Galactic disc is well known to
increase substantially with age; this is the so-called Age-Velocity relation,
and is interpreted as a ``heating'' of the disc as a function of time. We have
studied the heating of the disc due to giant molecular clouds and halo black
holes, using simulations of the orbits of tracer stars embedded in a patch of
the local Galactic disc. We examine a range of masses and number densities of
the giant molecular cloud and halo black hole perturbers. The heating of the
stellar disc in the simulations is fit with a simple power law, and we also fit
this form to the best determinations of Age-Velocity relation as derived from
stars in the solar neighbourhood for which ages can be reliably assigned.
Observationally, the Age-Velocity relation remains poorly constrained and its
determination is probably still dominated by systematic errors. Our simulations
confirm the well known results that there are insufficient GMCs to heat the
Galactic disc appropriately. A range of dark halo black hole scenarios are
verified to heat the stellar disc in the manner expected from analytical
studies, and they reproduce the observed ratio of the stellar velocity
dispersions. Simulations featuring a combination of giant molecular clouds and
halo black holes can explain the observed heating of the stellar disc, but
since other perturbing mechanisms, such as spiral arms, are yet to be
included,we regard this solution as ad hoc.
