It is shown that the north-south component of electric fields in the plasma sheet can be estimated uniquely from simultaneous measurements of plasma flow components (vx, vy) and the magnetic field components (Bx, By) in the equatorial plane. The estimated field strength ranges from 0.3 to 7.8 mv/m during substorms at geocentric distances beyond ~15 Re. During the magnetospheric substorm expansion phase, the north-south component of the electric fields is almost oppositely directed from what one would expect on the basis of projection of the general pattern of the ionospheric electric field to the plasma sheet along geomagnetic field lines (assumed to be equipotential). It is only during the recovery phase when both electric field patterns become similar. The result also suggests that strong fields occur preferentially near the plasma sheet boundary rather than near the neutral sheet region.