In a propagation treatment at all orders in the pump field and to first order in the probe and signal fields, for a homogeneously broadened line, we obtain three approximation levels for the Rayleigh-type optical mixing signal intensity: the first level is an analytical solution valid only for constant pump intensity; the analytical solution corresponding to the second approximation level considers pump propagation and yields good results for a large number ofcases; the third level is an exact but numerical solution. We present and compare the results obtained with the different approximations. © 1991 Optical Society of America.