The orbit of the Swedish satellite Viking was optimized for observations of plasma phenomena in the auroral acceleration regions, including auroral kilometric radiation. The auroral kilometric radiation (AKR) is the strongest electromagnetic radiation produced on auroral field lines, and the Viking satellite passed through the source region of this radiation on many orbits, as evidence by the very strong signals observed at the electron cyclotron frequency. For the first time, electron distribution functions were observed inside the AKR source region and immediately adjacent to the source region. The particle data show the presence of energetic electrons in and around the source region and also of upgoing ion beams of several keV energy in and only in the source region. Thus the particle data show clear signatures of parallel electric potentials both above and below the AKR source. The effect of a parallel potential below the source is to decrease the density of low-energy electrons and expand the upward loss cone, particularly at small parallel velocities. The reduction in the cold plasma density is clearly observed. Adjacent to the AKR source a clear loss cone is observed in the upgoing electron population. However, inside the source region the loss cone is filled or nearly filled. Our observations are in agreement with the cyclotron maser instability theory including the effect of a parallel electric field as discussed by Wu et al. (1982) and Pritchett and Strangeway (1985). ¿ American Geophysical Union 1990