Laser induced holographic grating relaxation (HGR), electro-optical (EO) and second harmonic generation (SHG) studies have been carried out in a guest/host system subject to an external electric field with and without the presence of a hole transport agent. The guest is NNI, which has moderate second order nonlinear optical (NLO) coefficient, the host is PMMA and the hole transport agent is DEH. In the presence of DEH, the lifetime of the cis isomer of NNI detected by using the HGR technique is lengthened by the application of an electric field of moderate strength. The field dependence effect is not observed when DEH is absent in the sample. The result is interpreted as due to the stabilization of the cis-isomer by the electric field polarized DEH. The data obtained from SHG and EO measurements appear to corroborate the HGR result.