We investigate the transient response of photogenerated carriers to an external electric field in bulk GaAs. The results of our Monte Carlo simulations indicate that the initial velocity rise times are a strong function of the carrier density. This is caused by a combination of the hot-phonon effect and the enhanced electron-hole scattering within the plasma. Contrary to some previous suggestions, the hot-phonon effect alone is insufficient to explain the initial velocity behavior seen experimentally. The steady-state velocity is limited by the electron-hole scattering.