  We study the Kondo effect in a quantum dot driven out of equilibrium by an
external ac field. The Kondo effect can be probed by measuring the dc current
induced by an auxiliary dc bias $V_{dc}$ applied across the dot. In the absence
of ac perturbation, the corresponding differential conductance $G(V_{dc})$ is
known to exhibit a sharp peak at $V_{dc}=0$, which is the manifestation of the
Kondo effect. In the equilibrium, there exists only one energy scale, the Kondo
temperature $T_K$, which controls all the low-energy physics of the system; $G$
is some universal function of $eV_{dc}/T_K$. We demonstrate that the dot out of
equilibrium is also characterized by a universal behavior: conductance $G$
depends on the ac field only through two dimensionless parameters, which are
the frequency $\omega$ and the amplitude of the ac perturbation, both divided
by $T_K$. We find analytically the large- and small-frequency asymptotes of the
universal dependence of $G$ on these parameters. The obtained results allow us
to predict the behavior of the conductance in the crossover regime
$\hbar\omega\sim T_K$.
