Specimens of Cu, Ag and Pt were irradiated in a fast neutron (f.n.) flux to a dose of about 2.5 × 1015n/cm2, yielding defect densities of about 4 × 10−7, and stored at 4.4 K. Specimens with the same prehistory were irradiated in a thermal neutron flux until the resistivity changes were the same as those recorded for the fast neutron irradiation. All samples were simultaneously annealed according to an isochronal programme.