The optical frequency doubling properties of silver, copper, and platinum nanoparticles in aqueous solution are studied via hyper-Rayleigh scattering (FIRS) spectroscopy. Substantial FIRS responses are observed for silver and copper particles. The response is attributed to enhancement due to resonance of the nonlinearly scattered light with the particles' surface plasmon absorption band. Platinum particles, which lack visible-region plasmon absorption, do not display detectable HRS signals. The largest signals from silver particles are observed under conditions of two-photon resonance, qualitatively consistent with the predictions of available theory; smaller yet still impressive signals are observed under pre- and post-resonant conditions. (C) 2002 Published by Elsevier Science B.V.