The S3-2 polar-orbiting satellite has provided simultaneous measurements of the transverse component of the ambient magnetic field, the trajectory component of the electric field, and the auroral electron flux during the very intense substorm period of March 26, 1976. Using orbits that traversed the midnight, dawn, and noon local time sectors of the low-altitude polar magnetosphere during extremely active times, we have calculated field-aligned current densities, compared tham with precipitating auroral electron fluxes, and by utilizing the direction (forward or backward) of the electric field component, identified the regions to which they map down. Our principal results from two such orbits, subject to the limitations of single-axis measurements, include the following: (1) The observed direction of the electric field component indicated closure of the major field-aligned current pattern by Pedersen current flow in the ionosphere, consistent with a magnetospheric current source. (2) Values of the height-integrated Pedersen conductivity, calculated from the gradients of the electric and magnetic field components, were typically in the range 10--30 &OHgr;-1 in the current-carrying regions during active times. At such times, particle precipitation on the nightside became the major factor in controlling the magnitude of the ionospheric conductivity, outweighing the influence that sunlight has in quiet times. (3) In regions of increased fluxes of precipitating electrons in the nightside auroral oval, current densities calculated from electron fluxes in the energy range 0.08- to 17-keV accounted for up to 50% of the upward currents, increasing to better than 70% during the most intense substorm period. (4) In each of the three auroral oval crossings on the nightside, an intense upward current sheet was observed at the equatorward slide of the westward electrojet, just poleward of the electric field reversal associated with the Harang discontinuity. (5) During the most intense current period, the region of field-aligned currents was compressed down to L shell values 2.2 to 5.1, within the trapping region, indicating that the equatorial source of these currents was the inner magnetospheric plasma inside the outer Van Allen radiation zone rather than the distant plasma sheet or the boundary layer.