  We consider the solvability of the Fokker-Planck equation with both
time-dependent drift and diffusion coefficients by means of the similarity
method. By the introduction of the similarity variable, the Fokker-Planck
equation is reduced to an ordinary differential equation. Adopting the natural
requirement that the probability current density vanishes at the boundary, the
resulting ordinary differential equation turns out to be integrable, and the
probability density function can be given in closed form. New examples of
exactly solvable Fokker-Planck equations are presented.
