The dynamics of bistable stochastic systems driven by oscillating fiel
ds is analyzed by numerically solving the Fokker-Planck equation. The
long time response is oscillatory in time and out of phase with respec
t to the field. The phase shift as a function of the noise D has a pea
k for weak fields and low frequencies if intrawell fluctuations are co
nsidered, in agreement with linear response theory. The amplitude has
a peak, but for a different value of D, even if intrawell fluctuations
are neglected. We also analyze the behavior for strong fields so that
linear response theory becomes questionable.