During irradiation, fast neutrons produce regions depleted of atoms. It is believed that these regions are responsible for most of the hardening observed after neutron irradiation. The present paper discusses the dissolution of the depleted zones during a post-irradiation anneal and the subsequent recovery of the yield strength. The zones dissolve by emitting vacancies and therefore have an activation energy made up of the sum of the vacancy binding energy to the zone and the vacancy migration energy. The resultant activation energy increases with recovery. For zirconium base alloys it was found that the activation energy varies from 49,700cal/mole at 10 per cent recovery to 61,600 cal/mole at 50 per cent recovery. Experimental data available indicate a similar increase in the activation energy with recovery.