The development of a kinematic model of the pro- and supination, that can be used to predict the influence of angulations of ulna and radius on the pronation and supination is based on the precise knowledge of the pronation and supination movement. We performed two parallel studies for examining the pronation and supination motion of the human forearm. The first experiment dealt with MRI-studies on 18 probands (36 examined forearms). As a result we observed an evasive movement of the ulna during the rotation of 7, 14 degrees medial. In order to prove whether the evasive movement was caused by a rotation of the humerus or by an evasion in the articulatio humeroulnaris, we carried out a second experiment, using 30 preparations. The measurement of the pro- and supination motion with a fixed humerus was expedited using a special experimental setup which guaranteed that the ulna could move freely. In all cases we found the same magnitude of the evasive motion of the ulna. Therefore we demonstrated, that the ulna performs an evasive motion during the pro- and supination motion of the forearm that influences the kinematic behavior of the pro- and supination motion significantly.