A method and plant for the introduction of a liquid into a molten mass under pressure. Such a method and plant are particularly, but not exclusively, suitable for the formation of a coating layer on a cable element having at least one conductor, the layer having an extruded thermoplastic polymer forming a continuous phase incorporating a dielectric liquid, and are useful, for example, in the production of an electric cable for the transportation and/or distribution of electrical power. The method includes the steps of bringing the liquid to a predetermined pressure greater than the pressure of the molten mass; feeding the liquid into a plurality of storage tanks, and injecting the liquid into the molten mass at an injection pressure equal to the above-mentioned predetermined pressure by means of a plurality of injectors in respective fluid communication with the plurality of storage tanks. Advantageously, this allows a substantial continuity of delivery of the liquid in a technologically simple way and at low cost, while ensuring a dispersion as uniform as possible of the liquid within the molten mass.