A fuel injection system of an internal combustion engine delivers fuel into a fuel rail via a high-pressure pump. The quantity of the delivered fuel is influenced by a quantity control valve operated by an electromagnetic operating device. A trigger signal supplied to the electromagnetic operating device is defined by at least two parameters, and a) in an adaptation method with the second parameter defined, at least one first parameter of the trigger signal supplied to the electromagnetic operating device is varied successively up to a final value at which a closing or opening of the quantity control valve is at least indirectly no longer or just barely detected, b) the first parameter is subsequently defined at least temporarily based on the final value, and c) the temporarily defined first parameter is adapted based on at least one prevailing operating variable of the fuel injection system or the second parameter is adapted based on at least one prevailing operating variable of the fuel injection system and of the temporarily defined first parameter.