We report the time behavior of nonlinear light diffraction driven by a square wave voltage in samples of polymer dispersed liquid crystals. The short peak (2–3 ms) in the leading edge of the signal is shown to be due to the occurrence of a transient amplitude grating in the sample. This effect is explained by the measured dependence of the transmittivity on the light intensity, reported here for the first time. A simple theoretical model gives a satisfactory account of the experiental observations.