The photochemical ring opening of spiropyrans in polymethylmethacrylate (PMMA) host films in the presence of a high-voltage electric field (2-5 MV/cm) leads to a photo-orientation of photomerocyanines. Since photomerocyanines have rather high first-older hyperpolarizabilities, this photo-orientation leads to second-harmonic generation (SHG). The second-order susceptibility d(33) Of PMMA films doped with 6-nitro-1’,3’,3’-trimethylspiro[2H-1-benzopyran-2,2’-indoline] (25% w/w) has been measured at 10 pm/V. If a PMMA film doped with spiropyran and preoriented by corona poling at 90 degrees C is irradiated in the UV, an SHG signal of photomerocyanine is also observed as a result of a memory effect of the orientation of spiropyran molecules. The photoswitching of this nonlinear optical property can be inverted by visible light irradiation. Only partially reversible switching is observed because of a gradual light-induced disorientation of photomerocyanine dipoles. Photochemical quantum yields of the spiropyran reversible arrow photomerocyanine reaction have also been determined in PMMA for one system. Dipole moments and hyperpolarizabilities of closed and open forms have been calculated by semiempirical and finite field methods for an analogous system.