We have probed the size dependency of the first hyperpolarizability (b) of copper nanoparticles by hyper-Rayleigh scattering (HRS). Our results indicate that second harmonic generation (SHG) originates predominantly at the surface of the nanoparticles as long as the size (d) remains small compared to the wavelength (k). However, volume contribution to the SH response due to the retardation effect becomes important when particle size grows beyond the 'small particle limit'. There is a significant dispersion in the b values of copper nanoparticles owing to the presence of the strong surface plasmon resonance (SPR) band. (C) 2009 Elsevier B.V. All rights reserved.