The impact ionization of excitonic states is studied in epitaxial GaN films and GaN/AlGaN quantum well structures. The investigation is carried out by optical methods involving the observation of quenching of the exciton photoluminescence under an applied electric field. It is found that impurity scattering rules the momentum and energy relaxation, rather than the acoustic phonon scattering. The effective mean free path of hot electrons is estimated. In GaN/AlGaN quantum wells the mean free paths of hot electrons appear to be an order of magnitude larger than those of GaN films due to the decrease in scattering probability of the electron in the two-dimensional case.