  In this paper, we extend our previous analysis (Lodato & Rice 2004) of the
transport properties induced by gravitational instabilities in cooling, gaseous
accretion discs to the case where the disc mass is comparable to the central
object. In order to do so, we have performed global, three-dimensional smoothed
particle hydrodynamics simulations of massive discs. These new simulations show
a much more complex temporal evolution with respect to the less massive case.
Whereas in the low disc mass case a self-regulated, marginally stable state
(characterized by an approximately constant radial profile of the stability
parameter $Q$) is easily established, in the high disc mass case we observe the
development of an initial transient and subsequent settling down in a
self-regulated state in some simulations, or a series or recurrent spiral
episodes, with low azimuthal wave number $m$, in others. Accretion in this last
case can therefore be a highly variable process. On the other hand, we find
that the secular evolution of the disc is relatively slow. In fact, the
time-average of the stress induced by self-gravity results in accretion
time-scales much longer than the dynamical timescale, in contrast with previous
isothermal simulations of massive accretion discs. We have also compared the
resulting stress tensor with the expectations based on a local theory of
transport, finding no significant evidence for global wave energy transport.
