Nucleon irradiation of metallic solid solutions may lower their resistivity by changing the degree of randomness (tendency to order or to cluster) or by a change of elastic moduli. Measurements of resistivity of brasses containing 10, 20, and 30% zinc have been performed at room temperature, at 80°K and at 4°K. The alloys were irradiated in a reactor at +50°C and at 80°K. It appears that the results can be best interpreted in terms of the formation of lattice defects upon low temperature exposure and increase of short‐range order upon irradiation at +50°C. The change of residual resistivity is negative while the change of resistivity due to thermal scattering is positive for the samples irradiated at +50°C. The former depends upon irradiation flux and upon annealing time in a different manner from the latter. Short‐range order appears to be produced also by proper heat treatment.