Actuator unit for a hydraulic vehicle brake system, with a master brake cylinder in which a first piston, which can be displaced by an actuator, is disposed, a first brake circuit which can be connected by a valve arrangement in a basic position to the master brake cylinder or separated from the latter thereby in an actuating position, and a device which can be connected to a hydraulic chamber of the master brake cylinder to simulate the brake pedal behaviour, wherein a second piston is disposed in a displaceable manner in the master brake cylinder, wherein the displacement of one piston causes the displacement of the other piston, wherein the device for simulating the brake pedal behaviour is switched to an active or inactive mode through the displacement.