The magnetic properties of grain-oriented 3 1/4% silicon-iron, as used in laminated transformer cores, are extremely sensitive to mechanical stresses. Power loss and magnetostriction (the cause of core vibration and noise) are most affected by compressive stresses. The theoretical effects of different types of stress on the domain structures of silicon-iron are correlated with measurements made on single laminations and transformer cores. The ways in which stresses can arise in cores and possible methods of avoiding or eliminating them in practice, are discussed. © 1974 Chapman and Hall Ltd.