Quenching of the exciton luminescence of GaN epitaxial films and GaN/AlGaN quantum wells in an electric field is observed and attributed to impact ionization of excitonic states by hot electrons. It is found that impurity scattering rules the momentum relaxation of electrons rather than the acoustic phonon scattering. The effective mean free path of hot electrons l(eff) is estimated. The value of l(eff) for quantum wells appears to be one order of magnitude larger than that for epitaxial films.