A fuel injection system, in which the injection quantity and the start of injection are controlled with solenoid valves, in view of engine-specific data and various parameters. Rotational-speed pulses are measured at the camshaft and/or at the crankshaft. Trigger times, which establish the start of injection and the injection quantity, are calculated on the basis of the rotational-speed pulses and a start-of-injection reference mark. Based upon an instantaneous rotational speed before the metering-in stage, an estimated value is determined, and based upon an instantaneous rotational speed during the metering-in stage, a control value is determined. The estimated value is compared to the control value and, if need be, the estimated value is adjusted.