A study by transmission electron microscopy was made of the microstructure of pure polycrystalline nickel foil after explosive‐shock loading and subsequent heat treatments between 600° and 780°C. In the instance of specimens shocked at 70 and 320 kbar, the deformation substructures resemble those normally caused by cold work, but the recovery process consists mainly in dislocation migration and recombination; no polygonization, nucleation, or grain growth are observed. Nickel shocked at 1000 kbar on the other hand, exhibits an extremely high density of dislocations, point defect clusters, and microtwins arranged in complex patterns. In this instance, heat treatment causes nonuniform polygonization, nucleation, and grain‐boundary migration. This results in a significantly lower dislocation density after annealing, particularly at the higher temperatures, and considerably more extensive softening than is the case for nickel shocked at lower pressures.