A circuit arrangement for the square-wave modulated high-frequency voltage operation of a discharge lamp, including means to control the duty cycle of the square wave, has a measuring circuit for detecting the ignition of the discharge lamp during each cycle of the substantially square-wave modulation and a timer circuit coupled to the measuring circuit for rendering the amplitude of the high-frequency voltage substantially equal to zero when a time interval has elapsed after the ignition of the discharge lamp, whereby the luminous flux of the discharge lamp fluctuates to a low degree only.