Abstract: To operate one or more serially connected low-pressure discharge lamps, tcally fluorescent lamps, a preheating circuit is provided to preheat the electrodes of the lamps (E1, E2, E3, E4), which changes from low-impedance to high-impedance state after the lamps have been preheated by controlling a semiconductor switch (Q3) in the heating circuit. In accordance with the invention, to eliminate reliance on the resistances of the lamp filaments themselves, which are subject to variation from lamp-to-lamp due to manufacturing tolerances, and later on, to changes due to aging of the lamp, and to provide for reliable switching of the semiconductor switch, a sensing impedance element (Z), which may be an ohmic resistor or a capacitor (Z', Z"), is serially connected to the switching path of the semiconductor switch (Q3) which, typically, is a field effect transistor (FET).