The invention relates to a device for actuating a control piston or nozzle needle (27) via a control chamber (26). The control chamber communicates via a control chamber line (25) with valve chambers (3, 22) of a 3/2-way valve (5). The valve body (6, 40, 51) of this valve is switchable by means of an actuator (11). By means of a restoring spring (14), the valve body (6, 40, 51) is acted upon such that a seat portion (7), embodied in the housing (4) on the valve body (6, 40, 51), is put into its valve seat (20). The valve body (6, 40, 51) of the 3/2-way valve (5) has a seat portion (7) and a longitudinal slide portion (8), as well as hydraulic faces (31,32), facing one another, that make the state of pressure equilibrium of the valve body (6, 40, 51) possible. The valve body (6, 40, 51) is moved to different stroke lengths (10; 23, 24) by means of an actuator (11).