Proposed is a circuit for starting internal-combustion engines by means of a sliding-gear starter motor (10), the circuit having a repeat-start device (18) which, on occurrence of a meshing block, repeats the start process after a predetermined period of time has elapsed. In order to detect the occurrence of a meshing blockage (failure of the starter to engage), the circuit is fitted with a voltage detector which senses the voltage drop at the battery (15) caused by the starter-motor start-up current on meshing. If there is no voltage drop, the starter (10) is switched off by a switch (20) in the repeat-start device (18) and is switched in again after a period of time determined by a time-switch in the repeat-start device (18).