BEGINNING 1966, after a 3 yr construction period by the Societe Tunisienne de l’Electricit6 et du Gaz (S.T.E.G.), the 450 km long 150 kV line Tunis-Sfax-Gafsa/Metlaoui, a German foreign assistance project, was activated to supply about half of the country with electric energy. The region represents one-fifth of Tunisia’s total agricultural production, onethird of her industry, another third of her tourism, and all mining of phosphate rock.* The high-voltage transmission line was envisaged as part of the country-wide transmission ring system into which power is fed by two plants, one from the North and one from the South; i.e. the existing power plant La Goulette outside Tunis, and the one at Ghennouch near Gabts now in process for intra-system completion by 1971/72. The system run Tunis~afsa~Metlaoui comprises the southeast section of the ring. Its basic function, thus, is not limited to supplying the areas it touches which principally are the four governorates Sousse, Kairouan, Sfax and Gafsa. The analysis focuses, however, on this region because also within the ring system, regional supply represents the predominant function of the Tunis-Gafsa/Metlaoui line [I]. Together with all power stations and low-voltage networks, the high-voltage transmission line is just one out of three essentials in an electric energy system. Its effects cannot be ascribed to the different system parts. Analysis of benefits was extended and applied to the system as a whole accordingly. A computer simulation model was used to investigate whether the realized system alternative materializes as one of minimal cost in view of existing technological and economic constraints. Simultaneously the computer program provides for sensitivity testing as to alternative transport capacities, discount and foreign exchange rates, and internal rates of return. Cost~~ctive~ss ~~~~s~ was applied to check the realized system alternative with regard to four socio-economic goals [2]. To appraise the primary benefits of the energy system over an expected service life of 30 yr for the high-voltage line, within framework of tested cost-effectiveness, plan horizon was tixed at 1995. Concerning secondary benefits and the simulation model, the appraisal was limited to 1985.