The roles of carrier-carrier interactions and non-equilibrium phonons on the ultrafast relaxation of photoexcited carriers in GaAs are examined. At low carrier concentrations, the e-ph interaction is the main energy loss channel for hot electrons, while at high carrier concentrations, the e-h interaction is the primary energy loss channel. This latter result follows from the high e-h scattering rate, the screening of the e-ph interaction, and the high efficiency of hole-phonon scattering through the unscreened deformation potential interaction. The electron energy-loss rates through the e-h interaction increases as the excitation energies and intensities are increased. In two-dimensional systems, the e-h interaction further complicates the problem since the transverse optical modes out are also driven out of equilibrium by their interaction with the holes.