  In superconductors where the coherence length is comparable to the Fermi
wavelength, the vortex viscosity depends on the velocity of the vortex, leading
to non linear equations of motion. The trajectories of vortices driven by a. c.
fields show a variety of behaviors as function of frequency. Finite
pertubations give rise to very long lived transients. The relevance of these
results to experiments in high-T$_{\rm c}$ superconductors is discussed.
