Nonlinear magneto-optical rotation is' investigated at high light Dowers wh
ere the rotation is significantly modified by ac Stark shifts. These shifts
are shown to change the overall sign of rotation for closed F --> F + 1 tr
ansitions compared to the low light power limit. The effect is demonstrated
by measurements in rubidium and density matrix calculations. The results a
re important fur applications of nonlinear optical rotation such as sensiti
ve magnetometry.