The photogalvanic effect and even-harmonics generation in a mesoscopic junction are calculated in diffusion approximation. The characteristic values of the corresponding components of current are 10-9 Å under the irradiation of a field with Eω ~ 1 V/cm, ω ~ 1 GHz. An a.c.-field effect on the conductance of a mesoscopic junction is also investigated, and it was shown that in a wide range of field parameters it could differ from that which the weak localization predicts.