The paper describes, briefly, the background to the El Chocon 500 kV transmission system, indicating the size and location of the generating plant together with the atmospheric and natural conditions prevailing along the line route to the greater Buenos Aires load centre, some 1040 km to the north east of the generating sources. The electrical requirements of the two physically independent but parallel overhead lines are discussed in terms of the required phase-conductor arrangement. The optimum economic conductor size is shown to co-ordinate with the requirements of radio interference and line reactance. The expected fault performance of the lines is examined, and the earthwire and tower-foot earthing requirements to give satisfactory lightning performance are outlined. The mechanical design of fittings, including joints, clamps, spacers and dampers is described. A section is devoted to the electrical-performance requirements of the insulation, mainly in terms of the required switchingsurge performance and radio-interference characteristics. The tests necessary to prove the performance of the ‘V’ string insulators in satisfying the above requirements are described. The type of suspension tower finally chosen, of guyed construction, is justified, together with a description of tower foundations, guy anchors and tower tests. The paper concludes with a discussion of tower-assembly and erection techniques and the problems relating to conductor stringing.