At present, practically nothing is known about the dynamic response of materials subjected simultaneously to an external force and a flux of neutrons. In an endeavour to elucidate some of the essential features of the deformation behaviour under theseconditions, we have recently performed a series of uniaxial tensile tests on pure copper and a CuCrZr alloy in a fission reactor at 363 and 393K. In the following, we first describe the experiments and then present results illustrating the build up ofstress as a function of concurrently increasing strain and displacement dose level. Results on both pre- and post-deformation microstructures determined on the unirradiated as well as irradiated specimens using transmission electron microscopy (TEM) arealso presented. In these experiments, the transition from elastic to plastic regime occurs without any transient such as yield drop. As a result of the combination of strain and radiation hardening, the rate of hardening in the plastic regime is found tobe enhanced. The rate of hardening, the maximum level of hardening and the magnitude of the total elongation achieved during an in-reactor test are found to depend strongly on the pre-yield dose level (i.e. the level of the neutron fluence received by thematerial prior to the on set of plastic deformation). Dislocation activities in these experiments are significantly reduced compared to that in the case of the deformation of unirradiated materials. Even in the absence of a yield drop, the plasticdeformation is found to be localized in the form of cleared channels. The phenomenon of cleared channel formation also appears to be affected by the pre-yield dose level. The evolution of these channels appears to control the uniform elongation and thelifetime of the specimens. The main implications of these results are briefly discussed.