NOVELTY - The circuit (4) has a return line (11) guided from an anode gas outlet of a fuel cell stack (1) to an anode gas inlet of a fuel cell stack via a recirculation pump. Valves (15, 16) are designed as rotary movable flaps and arranged in the return line in a flow direction of the anode gas in front or behind a recirculation blower (12) i.e. side channel blower. The valves are designed to close a cross section of the return line. Heating elements i.e. heating cartridge, are arranged in direct proximity to the valves. The valves are arranged in a common plane and actuated by a lifting magnet.    USE - Anode gas circuit for use in a fuel cell system i.e. proton exchange membrane- fuel cell system, which is utilized in a motor vehicle.    ADVANTAGE - The valves are arranged in the return line in the flow direction of the anode gas in front or behind the recirculation blower, thus avoiding freezing of the rotary components of the recirculation blower, and minimizing the number and the complexity of the components to be used in the circuit, and hence reducing the overall weight of the fuel cell system. The circuit prevents the intrusion of additional humidity into the recirculation blower, thus preventing the formation of condensate at the recirculation blower so that a reliable operation of the recirculation blower is guaranteed even when water freezes at temperatures below the freezing point, and hence reducing the complexity for providing a protective measure against freezing of the recirculation blower. The valves are arranged in a common plane and can be operated through the lifting magnet, thus achieving synchronous opening of the valves at the inlet and at the outlet of the recirculation blower. The magnet can be sealed against the inner side of the housing via bellows, thus reliably preventing the intrusion of hydrogen or water into the lifting magnet, and hence increasing the life cycle of the lifting magnet.    DETAILED DESCRIPTION - The lifting magnet can be sealed against the inner side of the housing via bellows. An INDEPENDENT CLAIM is also included for a method for activating and deactivating an anode gas circuit of a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of an anode gas circuit at a fuel cell system.    Fuel cell stack (1)    Anode gas circuit (4)    Return line (11)    Recirculation blower (12)    Valves (15, 16)