Two devices for the direct measurement of the anelastic properties of metals in a reactor are described. First, a resonant cavity system has been adapted for the in‐pile determination of dimensional changes, with application to strain relaxation measurements at and above room temperature. Second, an inverted torsion pendulum operated at 130 Hz at a constant amplitude has been devised for the study of internal friction in metals exposed to a fast neutron flux. The relevant detection systems and the mechanical arrangements for both apparatus are given, together with typical recorded curves.