We report on the optical parametric excitation of silicon nanomechanical resonant structures. The threshold laser power needed to set these structures into self-oscillation was estimated to be around 70 muW. We measured resonant frequencies of up to 38 MHz by optical excitation; this method should extend to much higher frequencies. Under optical amplification of motion, the spectral response at resonance narrowed to the equivalent of a mechanical quality factor >20 000 at room temperature. These structures act as frequency tunable passive modulators of light, requiring no additional drive. (C) 2002 American Institute of Physics.