1. Reactor irradiation stimulates the process of cyclic stress relaxation in the investigated steel. The radiation-induced increase of the degree of relaxation does not depend on the radiation flux in the region of its variation, and it is probably determined by the parameters of the neutron flux. Previous irradiation by 5·1022 neutrons/m2 (E⩾0.1 MeV) does not affect the relaxing ability of the steel. 2. The nature of the development of cyclic stress relaxation does not depend on the holding time in a cycle in the region of its variation, both under ordinary and under reactor conditions.