Modulation of light by light as stimulus has been explored by the use of liquid-crystalline (LC) materials with photosensitive moieties. Azobenzene derivatives with monomeric and polymeric forms were prepared, which show nematic (N) LC behavior in the trans form and no LC phase in the cis isomer. Photoirradiation of a very thin film of the liquid-crystalline azobenzene derivatives (similar to 200 nm) in the trans form resulted in the disappearance of the N phase because of trans-cis photoisomerization of each mesogen, and the N phase recovered quickly when the irradiated sample was kept in the dark in the whole temperature range where the azobenzene LCs showed the N phase because of cis-trans thermal isomerization. Time-resolved measurements by the use of a laser pulse (355 nm, 10 ns fwhm) revealed that the N-to-isotropic (I) phase transition took place in 200 mu s in both monomeric and polymeric systems. This response is at least 1 order of magnitude faster than the response of nematic LCs previously reported.(1,2) Tn the polymer system the N-I phase transition was also induced in 200 mu s even below T-g. The isotropic glass state induced by photoirradiation in the polymer sample below T-g was stable at room temperature for a long period even after the trans form was restored by the thermal back-isomerization.