We present numerical simulations of gamma-ray bursts arising from exte
rnal shocks in the impulsive and wind models, including a weak or a st
rong coupling between electrons and protons plus magnetic fields, and
analyze the burst features in each scenario. We follow the dynamics of
the ejecta and external medium into the late stages of deceleration i
n order to study the hydrodynamics of the remnant and the temporal and
spectral evolution of the afterglow. A brief comparison with the opti
cal and radio afterglows of GRB 970228 and GRB 970508 is made.