A combined X-ray diffraction and metallographic examination has been made of the structure of a metal subjected to cyclic stressing. The X-ray tests show that when a symmetrical cycle in tension-compression is built up slowly the grains break down on a widespread scale into disoriented elements. But when the same stress cycle is built up faster than a critical rate the disorientation is largely suppressed. The metallographic evidence indicates that there is a corresponding transition from a widespread to a highly localized distribution of the deformation zones in the grains, and the combined results show that there must be a 'delay period' before the activation of the average slip movement. It is found, further, that, once a given stress cycle in excess of the safe fatigue limit is built up, continued cycles cause further movements which lead to the fatigue crack in the deformation zones. The movements do not continue when the cyclic stress is replaced by a static stress equal to the peak of the cycle, and they are attributed therefore to an enhancing of the atomic mobilities brought about in the deformation zones by the alternations of stress. It is shown, moreover, that the movements do not necessarily cause local strain-hardening and therefore do not occur by the usual slip mechanism of plastic flow, but more likely by a viscous motion.