The paper describes a digital-computer programme for calculating lightning surge voltages in high-voltage power systems. The voltages due to an injected lightning current in a transmission line are calculated simultaneously in the whole system considered. The flashover characteristics of the suspension insulators, the surge impedance and earth resistance of the towers, the surge impedance for mutually coupled conductors, the surge capacitance for station equipment and the characteristic of lightning arresters are all taken into account. The programme may, for instance, be used to evaluate general rules for the location of lightning arresters as a function of a few important parameters for conventional station arrangement. It may also be applied to calculate the voltages and to determine the protection requirement necessary for each special case.