Ultrafast electron dynamics are compared for small (2.5- to 4.0-nm average diameter) Au nanoparticles in two different surrounding environments. In one case, an aqueous solution contains An nanoparticles (with average diameters of either 2.5 or 4.0 nm) embedded inside polyamidoamine (PAMAM) dendrimers. In the other case, hexanethiol-passivated (HT) Au nanoparticles, with an average diameter of 3.6 nm, are suspended in dichloromethane solution. Femtosecond two-color pump-probe spectroscopy is used to excite and probe the dynamics of the Au nanoparticles in the region of the surface plasmon resonance. The transient response is measured as a function of laser excitation fluence (J/cm(2)). For the metal-dendrimer nanocomposites, the transient response consists of a single-exponential decay that relaxes with a time constant of less than 1.2 ps and is due mainly to electron-phonon coupling. The relaxation time scale shows a weak dependence on excitation fluence but is essentially independent of the size of the Au nanoparticle embedded inside the dendrimer (for the size range investigated). In contrast to the metal-dendrimer systems, the transient response of the Au-HT in dichloromethane reveals both electron-phonon and phonon-phonon relaxation components, and the relaxation time scale shows a relatively stronger dependence on excitation fluence. A qualitative discussion is given for the different relaxation characteristics that are observed for the two Au nanoparticle systems.