Mechanical property measurements on neutron‐irradiated MgO single crystals show that hardening is caused by fast neutrons exclusively. Microhardness measurements, followed by etching to reveal dislocation distribution, permit an analysis of plastic flow even on samples which are so embrittled by the irradiation (with a dose of 1019nvt) that it has not been possible to bend them. As determined from 3‐point bending experiments on samples annealed at temperatures up to 1200°C, it does require an increasingly higher stress to form a dislocation (source hardening) as well as to move it through the damaged lattice (friction hardening). The type of defects responsible for such behavior are discussed.