A series of controlled experiments was performed to investigate the effect of various stress-relief annealing treatments on magnetic permeability and tendency to aging of grain-oriented electrical steel strips M5, 0.30 mm. The best quality is produced by stress-relief annealing in a continuous roller type furnace. However, wound transformer cores have to be stress-relief annealed in a batch type furnace. Stress-relief annealing in batch type furnaces demands an appropriate temperature treatment and a protective atmosphere. Special care should be taken to prevent contamination by carbon. Carbon content in an amount greater than 0.001 percent considerably deteriorates magnetic properties at low inductions and increases the tendency of grain-oriented electrical steel strips to aging. Plastic deformation of electrical steel strips occurring during manufacturing of magnetic circuits should be avoided as much as possible. Magnetic properties impaired by them can never be fully recovered by stress-relief annealing, and they remain deteriorated with the effect being higher for the higher degree of plastic deformation.