Matthiessen's rule has been tested by, determining the slopes of the electrical resistivity-temperature curves for wires of eight common metals and alloys in various states of deformation by wire-drawing. The results show that the slope is independent of deformation, i.e. the rule is true, to within 0�5 per cent. for nickel, copper (two purities), iron, and 80/20 brass, and to within 1 per cent. for aluminium. However, for 75/25 brass and an aluminium bronze, deformations corresponding to logarithmic strains of 2�3 decrease the slopes by 1 and 3 per cent. respectively. As an explanation of this behaviour, it is suggested that deformation causes an increase in the characteristic temperature.