The rabbit flocculus can be divided into five zones (zones 1, 2, 3, 4, and C2) with the use of acetylcholinesterase histochemistry. The projections of individual Purkinje cells in these zones to the vestibular and cerebellar nuclei were studied by using biocytin as an anterograde tracer. The zones were physiologically identified in terms of the Purkinje cell complex spike modulation occurring in response to optokinetic stimulation. In zones 1 and 3 neurons respond best to rotation about a horizontal axis that is close to perpendicular to the ipsilateral anterior semicircular canal, whereas in zones 2 and 4 neurons respond best to rotation about the vertical axis. Complex spike activity in zone C2 is unresponsive to optokinetic stimulation. Collectively, Purkinje cells of zone 1 projected to the ventral dentate nucleus, dorsal group y, and superior vestibular nucleus; Purkinje cells of zones 2 and 4 projected to the magnocellular and parvicellular parts of the medial vestibular nucleus; Purkinje cells of zone 3 projected to dorsal group y, ventral group y, and the superior vestibular nucleus; and Purkinje cells of zone C2 projected to the interposed posterior nucleus and dorsal group y. Some of the labeled Purkinje cell axons branched and innervated two nuclei. Branching axons from zone 1 either innervated both the ventral dentate nucleus and the superior vestibular nucleus or both dorsal group y and the superior vestibular nucleus. Branching axons from zones 2 and 4 innervated both the magnocellular and the parvicellular parts of the medial vestibular nucleus. Branching axons from zone 3 innervated both dorsal group y and the superior vestibular nucleus, or both ventral group y and the superior vestibular nucleus. Branching axons from zone C2 innervated both the interposed posterior nucleus and dorsal group y. Some of the target nuclei of the floccular Purkinje cell axons (e.g., dorsal group y and interposed posterior nucleus) project to the part of the inferior olive that, in turn, projects to the corresponding floccular zone, thus completing a closed pathway consisting of the inferior olive, the cerebellar cortex, and the cerebellar and vestibular nuclei. Other target nuclei (e.g., superior vestibular nucleus and medial vestibular nucleus) do not project back to the olivary subnuclei that innervate the flocculus and are part of an open olivofloccular pathway. An individual Purkinje cell thus can innervate a nucleus in the closed pathway as well as a nucleus in the open pathway.(ABSTRACT TRUNCATED AT 400 WORDS)