Permendur in laminated form is often used in magnetic cores of machines and transformers where it might be subjected to mechanical stress. Measurements have been made to determine to what extent the a.c. iron loss and magnetostriction are affected by such linear stress. Magnetic properties were found to be dependent on heat treatment over the temperature range 400°C to 900°C. The loss of material annealed above 710°C rose steeply when compressive stress up to 100 MN/m2was applied, whereas under tension it first dropped slightly and then rose steadily as the stress was increased to 300 MN/m2. The iron loss of material annealed below 710°C was high, but its stress sensitivity was low; there was a more significant drop in loss under tension and only a marginal increase under compression. The magnetostriction was found to decrease rapidly under tension and its stress sensitivity did not vary greatly with heat treatment.