Attempts were made to determine brainstem and cerebellar afferent and efferent projections of the superior vestibular nucleus (SVN) and cell group 'y' ('y') in the cat using axoplasmic tracers. Injections of HRP, WGA-HRP and [3H]amino acids were made into SVN and 'y' using two different infratentorial stereotaxic approaches. Controls were provided by unilateral HRP injections involving the oculomotor nuclear complex (OMC), the interstitial nucleus of Cajal (INC) and the deep cerebellar nuclei (DCN). Large injections of SVN almost invariably involved 'y' and dorsal parts of the lateral vestibular nucleus (LVN). Smaller injections involved central and ventral peripheral parts of SVN. Discrete injections of 'y' involved small dorsal parts of LVN. Afferents to SVN are derived mainly from the vestibular nuclei (VN) and parts of the vestibulocerebellum. SVN receives afferents: bilaterally from caudal portions of the medial (MVN) and inferior (IVN) vestibular nuclei and 'y'; contralaterally from ventral and lateral parts of SVN and rostral MVN; and ipsilaterally from the nodulus, uvula and medial parts of the flocculus. Purkinje cells (PC) in medial parts of the flocculus project to central regions of SVN, while PC in the nodulus and uvula appear to project mainly to dorsal peripheral regions of SVN. SVN receives sparse projections from the ipsilateral INC, the contralateral central cervical nucleus (CCN) and virtually no projections from the reticular formation. SVN projects via the medial longitudinal fasciculus (MLF) to the ipsilateral trochlear nucleus (TN), the inferior rectus subdivision of the OMC, the INC, the nucleus of Darkschewitsch (ND) and the rostral interstitial nucleus of the MLF (RiMLF). Contralateral projections of SVN cross in the ventral tegmentum caudal to most of the decussating fibers of the superior cerebellar peduncle and terminate in the dorsal rim of the TN and the superior rectus and inferior oblique subdivisions of the OMC; sparse crossed projections enter the INC and the ND. Cerebellar projections of SVN end as mossy fibers in the ipsilateral nodulus, uvula and in medial parts of the flocculus bilaterally. Retrograde transport from unilateral injections of the OMC indicate that afferents from SVN arise ipsilaterally from central and dorsal regions and contralaterally from dorsal peripheral regions. Ventral cell group 'y' receives small numbers of afferent fibers from caudal central parts of the ipsilateral flocculus. No fibers from ventral 'y' could be traced to other vestibular nuclei, the OMC or the cerebellum.(ABSTRACT TRUNCATED AT 400 WORDS)