An electrostatic electron energy analyser has been inserted for the first time in the outer region of the RFX reversed-field pinch experiment, in order to investigate and characterize the presence of a superthermal electron population. It has been found that these electrons carry most of the current density parallel to the magnetic field. The time evolution during a single discharge of the superthermal electrons current density and parallel temperature indicates that the distortion of the electron distribution function is stationary during the plasma current flat-top phase. The dependence of the superthermal temperature on the plasma parameters has been investigated by varying the plasma density, and a relationship with the ratio of the on-axis applied electric field to the critical electric field for runaway generation has been identified.