Unilateral lesions of the middle suprasylvian gyrus in the cat parietal cortex produce an asymmetrical vestibulo-ocular reflex such that responses to head rotation toward the side opposite to the lesion become weaker. Unilateral lesions of the superior colliculus in the cat also produce the same effect. In either case symmetrical responses are recovered within 2-3 weeks. These results are discussed in terms of displacement, by the unilateral lesion, of an internal reference used for directing behaviour within extrapersonal space. Relevance of this hypothesis to clinical symptoms observed in man after unilateral posterior parietal lesions is discussed, with particular emphasis on the unilateral neglect phenomenon.