The detection of the profile of the radial electric field of a tokamak by the perpendicular injection of a pulsed neutral beam into the plasma is analyzed. The method is based on the measurement of the toroidal precession velocity of the thus formed ion population. The toroidal drift is caused by the curvature of the magnetic field lines and by the electric field. For the separation of these two effects, the use of a beam with two energy fractions is studied. The application of this diagnostic scheme in a real tokamak is considered. 34 refs., 6 figs., 4 tabs.