Using ensemble Monte Carlo methods, coupled with a molecular dynamics (MD) approach for the carrier-carrier interaction, we investigate the ultrafast relaxation of photoexcited carriers in GaAs. The interaction of various scattering mechanisms and the dynamic screening of hot carriers in semiconductors is studied. At a density for which the GaAs is degenerate (in equilibrium), scattering out of the excitation volume is dominated in the initial tens of femtoseconds by electron-electron scattering, and the scattering rate increases with increasing density. This rate increase agrees both in magnitude and in density dependence with some recent experiments. The presence of electron-electron scattering modifies both the population transition rates and carrier densities in the satellite valleys, primarily by reshaping the energy distribution of carriers in the central valley. Intervalley processes also play a role in the initial decay and the same processes play a modified role in the picosecond-scale luminescence decay. The intervalley transition rates must be estimated carefully because the Γ-L population shift contains a significant fraction of electrons that reach the L valleys by way of the X valleys. The exchange effect further modifies the satellite valley populations and intervalley transition rates. It also reduces the rate for electrons to scatter out of the excitation volume.