Temporarily fixable positioning system (1) for secure, play-free fastening to the human body, preferably to the human skull has a base (4), a clampable ball-and-socket joint (3), a linear axis mechanism (2) fixed securely to it and a carriage (6) guided along the linear axis mechanism for receiving an actuator or sensor instrument (12). For intraoperative use, the positioning system is combined with a retractor (27). A head support (31) is provided for noninvasive diagnostic or therapeutic applications. The combination of the axial degree of freedom of linear axis (2) with the three rotational degrees of freedom of the ball-and-socket joint (3) allows four-axis, in situ positioning of the instrument on or in the skull, especially in the external auditory canal and an opened middle ear. The positioning system in combination with one of the holders enables precise positioning of an active end (14) of a guided instrument (12) relative to sensitive anatomical structures while avoiding risky relative movements.