  We quantify the effect of Gaussian white noise on fast--slow dynamical
systems with one fast and two slow variables, which display mixed-mode
oscillations owing to the presence of a folded-node singularity. The stochastic
system can be described by a continuous-space, discrete-time Markov chain,
recording the returns of sample paths to a Poincar\'e section. We provide
estimates on the kernel of this Markov chain, depending on the system
parameters and the noise intensity. These results yield predictions on the
observed random mixed-mode oscillation patterns. Our analysis shows that there
is an intricate interplay between the number of small-amplitude oscillations
and the global return mechanism. In combination with a local saturation
phenomenon near the folded node, this interplay can modify the number of
small-amplitude oscillations after a large-amplitude oscillation. Finally,
sufficient conditions are derived which determine when the noise increases the
number of small-amplitude oscillations and when it decreases this number.
