This proposal consists of a series of anatomic tract tracing studies of the topographic organization of the neuronal circuits involving the brain stem vestibular nuclei, the inferior olive and the cerebellar nodulus in the rabbit. These experiments are a direct extension of the PI's previous research and will complete characterization of the olivo-nodulo-vestibulo- olivary circuit with tracing methods. Anterograde and retrograde double tracer methods will be employed to test hypotheses about the topographic organization of three axonal pathways in these circuits. First, anterograde methods will be used to test the hypothesis that the topography of the nodulo-vestibular projection is congruent with the topography of olivo-vestibular and olivo-nodulus projections. Second, anterograde and retrograde tracing methods will be used to test whether crossed vestibulo- olivary and olivo-vestibular projects are reciprocal. Finally, anterograde and retrograde tracing methods will be used to determine whether uncrossed vestibulo-olivary projects are (1) a mirror image of the crossed vestibulo- olivary path and/or (2) a connection to the same olivary division that contributes the crossed olivo-vestibular projection. The data from these experiments will provide an anatomical model for the organization of these circuits that will be important for understanding the anatomic substrate for vestibular reflexes.