The specific aim of the proposed research is to study the distribution and sites of termination of primary and secondary vestibular fibers in the brain stem and cerebellum in the monkey and cat using anterograde and retrograde axonal transport technics. Primary vestibular fibers will be studied in monkeys in which small volumes of tritiated amino acids L-leucine (4,5-3H(N)) and L-proline (2,3,-3H(N)) have been injected into the ampullae of the semicircular canals, the utricle or the saccule; transport of the label centrally by cells of the vestibular ganglion will be studied in serial sections of the brain stem and cerebellum in autoradiographs. Secondary vestibular fibers in the brain stem and cerebellum will be studied in animals in which small volumes of tritiated amino acids are injected or iontophoresed into individual vestibular nuclei. Transport of the label by axons of secondary vestibular fibers will be studied in autoradiographs prepared from serial sections of the brain stem and cerebellum. Attempts also will be made to inject small volumes of horseradish peroxidase (HRP) into the nuclei of the extraocular muscles, the accessory oculomotor nuclei (i.e., the interstitial nucleus of Cajal, the nucleus of Darkschewitsch and the nuclei of the posterior commissure), and thalamic relay nuclei, which are considered to receive vestibular inputs. The retrograde transport of this enzyme to cell somata will be studied. Data obtained from anterograde and retrograde axonal transport studies will be compared and analyzed. It is possible that data derived from this study may have clinical significance in the treatment of vertigo and disturbances of orientation in three-dimensional space.