  The movement of a Brownian particle in a fluctuating two-state periodic
potential is investigated. At high transition rate, we use a perturbation
method to obtain the analytical solution of the model. It is found that the net
current is a peaked function of thermal noise, barrier height and the
fluctuation ratio between the two states. The thermal noise may facilitate the
directed motion at a finite intensity. The asymmetry parameter of the potential
is sensitive to the direction of the net current.
