  We consider the problem of two-level system dynamics induced by the
time-dependent field B={a(t)cos\omega t,a(t)sin\omega t,\omega_0}, with a(t)
\sim cn(\nu t,k). The problem is exactly analytically solvable and we propose
the scheme for constructing the solutions. For all field configurations the
resonance conditions are discussed. The explicit solutions for N=1,2 we
obtained coincide at \omega=0 in the proper parameter domain with predictions
of the rotating wave approximation and agree nicely with numerical calculations
beyond it.
