Axoplasmic transport techniques were used to determine the contribution of the ascending tract of Deiters (ATD) to the vestibulothalamic projection in cats. Large injections of HRP into the thalamus centered on the border region between the ventrobasal complex and the caudal ventrolateral nucleus resulted in bilateral retrograde labeling of cells in the vestibular nuclear complex and the nucleus prepositus hypoglossi (PH). Similar thalamic injections were also made in animals with extensive bilateral lesions of the medial longitudinal fasciculus (MLF) and the brachium conjunctivum (BC). HRP-positive neurons in these cases were localized principally to the ventral lateral vestibular nucleus and adjacent superior vestibular nucleus ipsilateral to the thalamic injection, evidence that vestibulothalamic neurons in these nuclei may project to the thalamus over the unlesioned ATD. Injections of [35S]methionine into the rostral vestibular nuclear complex in animals with MLF and BC lesions confirmed these findings, demonstrating orthograde transport of radiolabel in the ATD with termination in thalamus. These experiments document a contribution of the ATD to the ipsilateral vestibulothalamic projection; other sources of the vestibulothalamic pathway (PH, Y group) likely travel through projection systems destroyed in the lesions made in the present study.