Deals with experimental and theoretical investigations on the characteristics of low-pressure Ar+Hg discharges with very high electric current densities. The positive column of these discharge systems is studied under the following conditions: noble gas pressure 7.7 Torr; tube radius 1.6 mm; cold-spot temperature 313-53 K (corresponding to a saturation mercury vapour pressure of 6-88 m Torr) and electric current 30-600 mA. It is found that the V-I characteristic of the positive column will be positive if the value of the current density is above a certain critical one. A six-level model which can fairly describe the depleted low-pressure Ar+Hg discharges is developed. The output of the theoretical model gives the whole macroscopic plasma properties such as the electric field strength, the electron temperature, the energy transfers of the various collision processes and the radiation output, etc. The E-I and T_e-I characteristics calculated according to the model agree qualitatively with the measured results.