Two active experiments, a relaxation sounder and a mutual impedance probe, have been implemented on board Viking. These active measurements, together with measurements from the electron spectrometers, are used to determine the plasma density in the cusp region, up to 13,500 km, the apogee of Viking, as well as in the sources of the auroral kilometric radiation (AKR). When active experiments are switched on in the cusp region, several plasma resonances are detected; they correspond to zero group velocity waves, indicative of a characteristic frequency of the plasma, namely the plasma frequency, and the fqn (n=1, 2, 3,...n) series, above fuh, the upper hybrid frequency. The fqn series allow an independent estimate of the plasma density. Plasma density in the cusp proper is found to be much larger (>100 cm-3 in some cases) than in the adjacent regions. An attempt is also made to estimate the respective densities and temperatures of the various components of the plasma. In contrast, auroral regions are low-density regions (fpe/fce≪1). The active sounding of the plasma by a relaxation sounder gives a resonance at fuh, which allows an estimate of the plasma density. Low-frequency whistler mode emissions are commonly observed in the night sector. Their upper cutoff frequency has often been used for estimating the plasma frequency. Active experiments are used to test this method, which is shown to be valid most of the time in the high-altitude auroral region, yet also sometimes misleading in other regions. Once the estimate of the density through the upper cutoff frequency of the hiss is validated, it can be used to follow with a good time resolution the sharp density variations experienced as Viking crossed AKR sources. It is found that the density within AKR sources is of the order of even less than 1 cm-3. ¿ American Geophysical Union 1990