A method for controlling an externally excited electric machine to boost regeneration of a NOx storage catalyst which is part of an assembly made up of an internal combustion engine, the externally excited electric machine which is connected to the internal combustion engine in a manner allowing the transmission of torque and which has a rotor including a rotor winding and a stator, and the NOx storage catalyst which is disposed in an exhaust-system branch downstream of the internal combustion engine, the rotor winding of the electric machine being energized with a preexcitation current as a function of an operating state of the NOx storage catalyst.