A memory state can be induced in PDLC films through the application of a strong D.C. voltage. This memory state is due to the onset of an internal polarization field, which keeps the film in a high transparent state. This polarization field remains in the film from hours to weeks, depending on the temperature at which the external polarizing field is applied. We have found the way to measure such an internal field by observing the electrooptical response of the film, under a Low frequency driving voltage. Due to the presence of the polarization field the light transmission obtained during the positive half period of the driving voltage is not equal to that observed in the negative half period. The polarization field corresponds to the D.C. bias voltage which is needed to equalize the light transmission in the two half periods. The time dependence of the polarization field has been determined.