The main information processing part of the cerebellum is its cortex, and its sole output is via the Purkinje cells which directly inhibit the neurones of the cerebellar nuclei. The cortico-nuclear pathway is thus central to cerebellar operations. An important organizational principle of the cerebellum is a division into a series of rostrocaudally-oriented olivo-cortico-nuclear modules. Some correspondence between the spatial location of these cortical component of these modules (zones) and the distribution of Purkinje cells that express zebrin II has also been found. Detailed electrophysiological mapping studies suggest that the cortical zones can be split into smaller units. The aim of the present investigation is two-fold: 1. To study the topographical organization of the interpositus nucleus in relation to its anatomical connectivity, zebrin II immunocytochemistry and physiological responses at this subzonal level of organization. 2. To examine the relationship between Purkinje cell and corticonuclear activity within the same module target. In barbiturate (60 mg/kg i.p) or urethane (1.3 g/kg i.p) anaesthetized adult Wistar rats we used percutaneous electrical stimulation of the ipsilateral forelimb, tail and different parts of the ipsilateral hindpaw to evoke climbing fibre field potentials in the C1 zone in the paramedian lobule and copular pyramidis. In anatomical studies, the evoked responses were used to guide nanoinjections of bidirectional tracer (fluorescent beads and BDA) into the cortex and after a survival period of 7 days the animals were deeply anaesthetised and the brains were removed for histological analysis. Injection sites were mapped relative to zebrin II stripes; beads-labelled olive cells were mapped onto transverse and horizontal maps of the inferior olive; and BDA-labelled Purkinje cell terminals were mapped onto a 3D template of the cerebellar nuclei. In physiological studies, simultaneous cerebellar cortical and nucleus interpositus anterior recordings were carried out in response to electrical stimulation of the forelimb and hindlimb in areas known to receive cortico-nuclear projections from the C1 zone. Preliminary anatomical results suggest that a detailed subzonal topography is present within the C1 zone in which climbing fibre responses, zebrin stripes and olivo-cortico-nuclear connections are correlated. Our physiological data demonstrates that evoked positive field potentials in the nucleus interpositus anterior and climbing fibre potentials in the cerebellar cortex are positively correlated. Preliminary experiments of single unit recordings suggest a complex pattern of interactions between Purkinje cells and cerebellar nuclear cells within the same module.