Non - equilibrium tunneling current in a Kondo system subject to both constant and alternating electric fields is studied. First, a time - dependent Schrieffer - Wolff transformation is performed, mapping the time - dependent Anderson Hamiltonian onto a Kondo one. Then, a perturbation expansion in powers of the coupling strength is carried out up to third order, yielding a simple analytical expression for the tunneling current. It is found that the Kondo effect enhances the first harmonic as much as the dc one, leaving the other harmonics relatively small. This result is shown to be valid also below the Kondo temperature.