In a process for regulating the light intensity of gas-discharge tubes containing a heated cathode and an anode separated from the cathode, a voltage sufficient to maintain the gas discharge and which repeats itself during a periodic time less than the inertial time of the eye, and which vanishes at least once per period is applied between cathode and anode. The value of this voltage is increased to the value required to initiate the gas discharge at least once per period with a delay related to the periodically repeating times at which the voltage vanishes. The gas discharge is maintained until the next time at which the voltage vanishes and the light intensity is modified by lengthening or shortening the corresponding delay time. In the switchgear for regulating the light intensity of gas discharge tubes, a voltage source (16) which produces a periodically vanishing voltage is incorporated between anode and cathode of the gas-discharge tube (10), and the cathode is connected to a heating current source. A transformer (25) with high transformation ratio or a pulse transformer (14) whose primary winding (30) is connected to the output of the pulse generator (17, 28) actuated at the corresponding delay time is incorporated between the electrodes (12, 13) of the gas-discharge tube (10). The switchgear contains a stage which periodically breaks the electrical circuit between the electrodes (12, 13).