An HRP study of the frog's superior olivary nucleus (SON) revealed that it (1) receives reciprocal tonotopic projections from dorsal medullary nuclei and principal nuclei of the torus semicircularis (TS) bilaterally; (2) receives bilateral projections from caudalis nuclei and brainstem reticular nuclei, and unilateral projections from the ipsilateral ventral tegmental nuclei and the contralateral SON; (3) is reciprocally connected with the ipsilateral laminar and magnocellular nuclei of the TS, dorsal tegmental nuclei and the posterior thalamic nucleus; (4) projects directly to the ipsilateral central thalamic nucleus.