A long-period magnetotelluric study was carried out in the central Andes between latitudes 19.5°S and 21°S along two almost parallel profiles of 220 and 380 km length, respectively. The investigation area extends from the Pacific coast to the southern Altiplano Plateau in the back arc of the South American subduction zone. The main geoelectrical structure resolved is a broad and probably deep-reaching highly conductive zone in the middle and deeper crust beneath the high plateau. Although the data show deviations from two-dimensionality, a two-dimensional approach is justified for large parts of the profiles. Sensitivity studies were carried out in order to constrain the depth extent. Another electrically conductive structure was resolved in the middle crust of the Chilean forearc, thought to be connected with the Precordillera fault system. The Andean Continental Research Program (ANCORP) seismic reflection profile, carried out along the same line at 21°S, revealed highly reflective zones below the Altiplano, in good correlation with the upper boundary of the Altiplano conductor. This highly conductive domain also coincides with low seismic velocities and a zone of an elevated v_p/v_s ratio and, although not well resolved, with low Q_p seismic quality factors. Taking into account the enhanced heat flow and a derived temperature model, the most probable explanation lies in the assumption of granitic partial melts. The good conductor below the volcanic arc which was found in regions farther south at 22°S gradually vanishes toward the north; this is consistent with the results of seismic tomography concerning Q_p values and a gap of recent volcanism.