Alternate Langmuir-Blodgett (LB) films consisting of a ruthenium(Il)-bipyridine complex and ammonium amphiphiles exhibit a strong second-harmonic generation (SHG) on irradiation with nanosecond neodymium-doped yttrium aluminium garnet 1064 nm laser pulses. We successfully demonstrated that the SHG intensity from the LB films was reversibly modulated when the films were exposed to nanosecond 355 nm laser pulses. The switching time for the optical modulation of SHG was determined to be less than 2 ps from a subpicosecond dye laser pump-probe SHG experiment. The mechanism for the optical modulation of SHG was ascribed to a change in the molecular hyperpolarizability of the ruthenium complex on going from the ground state to the excited state.