For economical insulation co-ordination in transmission and substation equipment, it is necessary to accurately predict the lightning surge overvoltage that occurs in an electric power system. The lightning surge response of transmission towers is one of the main factors that influences the magnitude and the waveshape of the surge arriving at a substation. The paper presents a tower model, based on a finite-difference transmission-line model, that can be used with EMTP. Computation results obtained, for a typical double-circuit 500 kV tower, are in good agreement with field test measurements.