Irradiation of a superconductor by an electromagnetic field with a frequency OJo larger than twice the energy gap (order parameter) decreases the order parameter A. We obtain the quasiparticle distribution function n and the dependence of the order parameter on the power of the electromagnetic field P by solving numerically the kinetic equations for n and A in the steady state. We take OJo/Ao = 2.1, 8, and 20, where Ao is the equilibrium value of the order parameter at T = 0 K. In the examples considered the dependence of A on the pumping power P becomes double-valued above a critical power. We allow phonons also to be out of thermal equilibrium. To discuss the stability of the steady state thus obtained, we derive kinetic equations for small deviations of the quasiparticle distribution fimction and the order parameter from the stead), state 6n and 6A by means of the nonequilibrium Green's function theory. Assuming 8n, 8A oc exp (ikr-at), a(k) is computed using 8ns/SA, where ns is the steadystate quasiparticle, distribution function for arbitrary A. It is concluded in general that the steady state on the upper branch (A > AcQ is stable with respect to both spatially homogeneous and inhomogeneous fluctuations, and the lower branch (A < At2) is unstable; At2 is the value where the upper and lower branches of A(P) coalesce.