Approximately 1200 hours of high-atitude balloon measurements of atmospheric vertical electric fields and conductivity have been used to derive the UT variation of the ionosphere-to-ground vertical current. During geomagnetically quiet days the well known UT modulation, with a minimum at 4--6 UT and a maximum near 19 UT, is observed. During distributed days, the dawn-dusk potential drop across the polar cap clearly influences the vertical current, as was predicted by Park and Dejnakarintra (1977).