Summary The effect on Young's Modulus of copper of small amounts of plastic extension (1–8%) and subsequent low temperature annealing (50–100°c) has been studied. The results confirm previous work that there is a reduction of about 11% in the modulus, the greater part of which is recoverable after prolonged annealing at 100°c. The reduction is attributed to the elastic oscillation of the relatively mobile screw components of dislocation loops, recovery being caused by the annihilation of such components in pairs by cross-slip, as suggested by Mott (1952). In an analysis of the recovery process it is assumed that the activation energy required to move the two screw components together is not unique but is distributed in a Gaussian manner about a mean. This mean value is found to be 25 000 cals/mole for O.F.H.C: copper extended by 1% and 21 500 cals/mole for the same mgterial extended by 8.4%.