After lesion of parts of the cerebellum of cats, particularly of the lobus posterior medianus, nystagmus was observed in abnormal positions of the head, particularly in supine position with the vertex downward. A weak nystagmus in normal position of the head was increased, sometimes also its direction was changed in abnormal positions; or nystagmus appeared in abnormal positions only. This phenomenon was transient. Electrolytic lesions of the nuclei tecti had a similar effect. In analyzing the receptor mechanism, neck reflexes and retinal reflexes could be excluded. Bilateral labyrinthectomy abolished this effect of position, while a weak spontaneous nystagmus could persist. It is assumed that the appearance of positional nystagmus following cerebellar lesions is a phenomenon of release of parts of the vestibulo-ocular reflex arcs, since it is associated with increase of the exptl. post-rotatory nystagmus. The expts. suggest that the existence of positional nystagmus should routinely be tested in suspected cerebellar lesions.