An electrohydraulic brake system having a muscle-force-actuatable auxiliary brake and an external-force-actuatable service brake includes operating mode switchover valves triggerable by an electronic control unit for switching from the service braking state to the auxiliary braking state. A piston/cylinder unit with a pressure medium connection between a master cylinder and a wheel brake which is interrupted by the switchover valves to simulate the pedal travel in the service braking state. The piston/cylinder receives the pressure medium displaced in the master cylinder by actuation of the brake pedal by the driver's foot. The piston/cylinder unit can be hydraulically blocked in the auxiliary braking state so that pressure medium positively displaced in the master cylinder is available to the greatest possible extent for building up pressure at the wheel brakes. The subjection of the piston/cylinder unit to pressure medium is controlled via one of the operating mode switchover valves.