A strong pump-power dependence of the Rayleigh-type optical-mixing signal for a water solution of malachite green is reported. Experimental evidence of an exponential pump-power dependence is given. The essential characteristics of the pump-power dependence and the frequency spectrum of the nonlinear signal are qualitatively reproduced by a theoretical model where coupling between pump, probe, and signal fields and propagation effects are considered. © 1988 The American Physical Society.