An internal combustion engine ignition system includes an output switching transistor (7), the emitter-collector path of which is in series with the primary winding (5) of an ignition coil (6). To attain a constant amount of ignition energy in the ignition coil (6), the control current for the base of the output switching transistor (7) is supplied by a constant-current source (9), the level of the constant control current being selected such that while the ignition energy is being stored in the ignition coil (6), the saturated operation of the output switching transistor (7) is maintained.