In a fuel cell system having a gas generation system for providing hydrogen from a carbon- and hydrogen-containing medium for a fuel cell unit, means are provided for adjusting pressure in the gas generation system and/or mass flow through the fuel cell unit, upstream of the fuel cell unit. In a transition from a part-load range to a full-load range, the pressure can be at least briefly reduced, and in a transition from a full-load range to a part-load range, the pressure can be at least briefly increased.