A description of plasma properties within the sources of the Auroral Kilometric Radiation (AKR) is given. It is based on data collected during ~50 AKF source crossings in the altitude range between 4000 and 9000 km by the Swedish spacecraft Viking. The following results are obtained. (i) The frequency of the lowest frequency peak of the AKR fpeak is found to be very close to fce, the electron gyrofrequency ((fpeak-fce)/fce≤0.08), on the average, (ii) The lower cutoff frequency fLC is on the average at fce((fLC-fce) /fce≂0), (iii) in the sources the density is typically less than 1.5 cm-3, which is of the order of the density of hot electrons and (iv) the source is located within an acceleration region, as evidenced by electrons accelerated above and ions accelerated below. ¿American Geophysical Union 1991