A device and method for adjusting a mounting position of a window lifter guide rail of a motor vehicle window, particularly in a frameless motor vehicle door. The device comprises a locking member which includes a lower member, an upper member, and a screw that extends through the locking member. The screw is in engagement with a nut that may be rotated to adjust the mounting position of the guide rail in a vertical direction. The upper and lower members may also be moved relative to one another to adjust the mounting position of the guide rail in a horizontal direction.