The invention relates to a two-wire switch (1 ) comprising a control unit (2) for controlling the functions of the switch (3), the switch (3) proper consisting of at least one voltage-controlled semiconductor switch (3; 31, 32), and comprising additionally a power source (4) of the control unit (2) such that is disposed to be supplied with electric power from the voltage (Vk, Vh) across the switch (3), both with the switch (3) in non-conductive state and in conductive state. In accordance with the invention, the two-wire switch (1) comprises a measuring and control circuit (5) for measuring the dropout voltage (Vh) across the switch (3) when the switch (3) is in conductive state and for regulating the resistance (Rk) of the switch (3) simultaneously with the dropout voltage (Vh) on the basis of the measured dropout voltage (Vh). The invention yields the advantage of low switching losses and of optimised voltage across the switch, i.e. voltage level. The voltage level is determined by the technique used for generating the operating voltage of the control unit.