  The tails of prehistory probability density in nonlinear multistable
stochastic systems driven by white Gaussian noise, which has been a subject of
recent study, are analyzed by employing the concepts of nonstationary optimal
fluctuations.
  Results of numerical simulations evidence that the prehistory probability
density is non-Gaussian and highly asymmetrical that is an essential feature of
noise driven fluctuations in nonlinear systems. We show also that in systems
with the detail balance the prehistory probability density is the conventional
transition probability that obeys the backward Kolmogorov equation.
