Polycrystalline specimens of copper have been fatigued in reverse bending, in a vacuum at 405°C at a maximum strain amplitude of 0.044%. A specimen tested to 0.07% of its expected life gives neutron scattering which is isotropic and identical to that of an unfatigued specimen, and which can be attributed to surface oxides. A specimen fatigued to 25% of its expected life gives a larger scattering, which is anisotropic. This is attributed to faceted pores at grain boundaries resulting from the high-temperature fatigue. The quantitative results indicate the presence of pores smaller than those so far observed in the electron microscope.