The conditions are investigated for the preparation of ultradispersed magnesium nitride (UDMN) from elemental magnesium and nitrogen by the electric-are low-temperature plasma (LTP) technique. Thermodynamic calculations are carried out with the objective of finding the values of the equilibrium constant for the synthesis reaction of magnesium nitride from its elements in the temperature range 298-1800 K as a function of the aggregation state of the components. By means of a "cold" wall (CW) plasma-chemical reactor (PCR), ultradispersed product (UDP) is synthesized with a specific surface of 33 m(2) g(-1) containing up to 56% of Mg3N2; the use of a "warm" wall (WW) PCR allows the preparation of UDP with a Mg3N2 content of up to 73% and a specific surface reaching 184 m(2) g(-1). The dominant role of the radial temperature gradient in the PCR in the synthesis of UDMN with maximal degree of nitride formation and maximal specific surface values is experimentally demonstrated. The UDMN exhibits a high chemical activity; heat-treatment at 900 K in an inert atmosphere decreases substantially its chemical affinity to water vapour and oxygen in the air.