Single-crystal nickel was irradiated with 590 MeV protons to 10(-1) dpa at room temperature. Irradiated and unirradiated tensile samples were deformed and relaxation tests were performed at temperatures between 77 and 423 K. The tests show a strong temperature dependence of the flow stress for samples irradiated to 0.1 dpa as compared to the unirradiated case. Unirradiated and irradiated deformed microstructures were investigated by transmission electron microscopy. The initial plastic deformation of the samples irradiated at 0.1 dpa shows strain localization in the form of defect free channels, over the temperature range from 77 to 293 K. Deformation processes are analysed through the determination of the activation energies of the deformation mechanisms as deduced from relaxation tests. The activation energy has an approximate value of 0.5 eV in unirradiated samples. In the irradiated samples it is suggested that multiple deformation processes are operative in the temperature range from 77 to 423 K.